The Fun Differential Rolls on in Seattle

This has not been the year for the AL West. With the reigning World Series champion Rangers sitting below .500 amid a string of injuries, the Astros’ core succumbing to age, and the Angels and A’s sitting at rock bottom, one of baseball’s stronger divisions over the past few years has become its weakest. Just one team has a winning record: the Seattle Mariners. At 43-31, the Mariners hold an 8.5-game lead in the West, even as some of the underlying numbers indicate the team isn’t as good as its record suggests. Seattle has overperformed its Pythagorean record by four wins and its BaseRuns record by two, and its run differential is by far the worst among division leaders. But this kind of thing is nothing new for this organization.

The Mariners are currently enjoying their fourth consecutive year of contention, falling short of a Wild Card spot in 2021 and ’23 and snapping their two-decade playoff drought in ’22. In each of these seasons, they’ve pulled out wins in close games like no other club, and manager Scott Servais has pointed to the poise and experience with which his team handles tight matchups. Famously, after a 2021 road trip where the Mariners went 6-2 despite being outscored by their opponents, Servais introduced the term “fun differential” to evaluate the team rather than its relatively poor run differential. Three years later, with a new group of players, the fun differential is still elite.

Naturally, in order to win a lot of one-run games, you need to play in a lot of one-run games. One of the best ways to do that is to play plenty of low-scoring affairs, when neither team scores enough runs to pull away from its opponent. And indeed, the Mariners rank in the bottom third of the majors in both runs scored and allowed. The first factor that puts them in so many tight games is the strength of their starting rotation, which has been among the best in baseball by both volume and efficiency. As a squad, they rank eighth in ERA- and FIP-, and second in innings per start; they’re one of just two teams to convert quality starts over half the time. While none of their starters are individually dominating the leaderboards, the depth they have is nearly unmatched. The Mariners are one of three teams (along with the Phillies and Yankees) with four qualified starters with an ERA- of 95 or lower, and even Seattle’s fifth slot (with starts made by Emerson Hancock, Bryan Woo, and Jhonathan Diaz) has pitched to a 3.25 ERA. In short, they’re the only team in the league that can expect to have good starting pitching every single night.

On the flip side, Seattle’s offense has taken a significant hit from last year. Lineup mainstays like J.P. Crawford, Julio Rodríguez, and Cal Raleigh have regressed this season, though Rodríguez has turned things around over the past month. Many of the hitters Seattle added during the offseason have underperformed as well. Returning fan favorite Mitch Haniger has been below replacement level, and Jorge Polanco and Mitch Garver are each hitting below the Mendoza line.

With an excellent rotation and below-average hitting, the Mariners have the recipe for low-scoring games, but there’s another factor here as well: their home field. T-Mobile Park has been regarded as a pitcher’s paradise since its opening 25 years ago, but it’s been even more unfavorable for hitters in 2024 than in previous years. Statcast’s single-season park factors view it as, by far, this season’s most pitcher-friendly park, with a factor of 87; it has had scores between 92 and 96 for the past half-decade. The end result is that nearly a third of Mariners games have been decided by a single run, one of the highest marks in the league.

Playing in a lot of one-run games is one thing, but winning them is another. The Cubs and Athletics, the only teams with more one-run contests, each have losing records in such games. But the Mariners combine quantity with quality, having the most one-run wins while placing second to the Rays in one-run winning percentage. In contests decided by multiple runs, the Mariners are 26-24 — their .520 win percentage in such games is shockingly close to their .527 Pythagorean record — but one-run wins have vaulted them to a dozen games above .500. Some of these wins have come in dramatic fashion, as their five walk-offs are tied for the league lead. The Mariners have been far from an offensive powerhouse, but all year the bats have come alive when it matters most.

These splits are staggering. In low leverage, the Mariners are one of worst-hitting teams in the league. But when the stakes are highest, they collectively produce like a top-15 hitter in baseball. However, the eye-popping 144 wRC+ figure in high-leverage spots comes with a .377 BABIP – more than 40 points higher than any other team in that split. Come year’s end, that number will certainly be lower than it is now, but looking underneath the hood, Seattle batsmen have still been hitting better in high leverage than low leverage. Their walk rate is three points higher and strikeout rate three points lower in such situations, and their hard-hit rate is also modestly higher.

While Mariners hitters might not be able to forever continue their dominance in dramatic moments, the production they are getting from their bullpen, the other component of their success in one-run games, is far more sustainable. Despite some confusing trades, strong relief pitching has been a strength of recent Seattle squads. The organization has a knack for finding, acquiring, and developing under-the-radar relievers.

Andrés Muñoz is enjoying his first full season as Mariners closer, but he hasn’t been deployed solely in ninth-inning save situations. In fact, only half of his appearances have begun at the start of the ninth inning. He’s been called upon for a couple of extra-inning appearances, but his most notable work has come when he’s inherited a dirty eighth inning and converted a four- or five-out save. Muñoz has recorded more than three outs in seven games, second to Mason Miller among full-time closers, and in those games, he hasn’t surrendered a single run. Servais has consistently picked the right time to get his relief ace onto the mound, as Muñoz has the highest average entrance leverage index in the league.

Veteran reliever Ryne Stanek and 31-year old breakout Tayler Saucedo, who each rank above some closers on the leverage index leaderboard, have mostly handled set-up duties ahead of Muñoz. The two of them complement each other well, as both Stanek, a righty, and Saucedo, a lefty, have significant platoon splits, and Servais shrewdly deploys them based on matchups.

Among Seattle’s lower-leverage options, former starters Austin Voth and Trent Thornton have hit their stride coming out of the bullpen; the pair lead the staff in relief innings while effectively keeping runs off the board.

It would be easy to chalk all of this up to luck, even within the context of the other recent Mariners teams. Their offense has less thump than it has in previous years, and their bullpen is more reliant on high-leverage studs than an entire stable of them. Yet, they still have the ingredients that have made them so successful in tight games, even if the recipe is a bit different. Besides, maybe a slight variation is a good thing. After all, in recent years the best the Mariners could do was secure one AL Wild Card berth. Now, for the first time in their fun differential era, they are in position to ride their recipe for success all the way to a division crown.