More on the topic of xWOBA under performance, here is a link from the Cardinals perspective on their woes this season:
https://www.vivaelbirdos.com/2023/6/18/23763490/some-xwoba-underperformance-goes-beyond-bad-luck
Here are some key quotes:
1)
"Fly balls hit to the middle of the park have a tendency to fare much better in xwOBA than they do in wOBA. Across, the league straightaway fly balls have a .520 xwOBA this year, which is fantastic, but those same fly balls only have a .312 wOBA, which is slightly below the .318 league average wOBA... If a hitter tends to hit a well above league average amount of straightaway fly balls, then he’s likely to underperform his xwOBA by a decent margin. That’s not bad luck. That’s simply sub-optimal batted ball direction."
2)
"While xwOBA comes in much higher than wOBA on straightaway fly balls, xwOBA drastically undervalues pulled fly balls. The league wOBA on pulled fly balls this year is .866 which is nearly 200 points higher than the league xwOBA on pulled fly balls (.672).
The moral of the story here - if a hitter pulls a large chunk of his fly ball and is overperforming his xwOBA, you can’t just chalk it up to good luck."
3)
"xwOBA is focused on how hard a hitter hits the ball and the angle at which the ball leaves the bat, and while those things are hugely important, speed makes a difference too. Speed can be the difference between an out or a hit on a weakly hit grounder. It can be the difference between a double and a triple on a ball in the gap. It can be the difference between a single and a double on a ball that’s somewhere not quite in the gap but not quite at the fielder either.
Players who are slow can often underperform their xwOBA for that very reason and players who are fast can overperform it by stealing hits and extra bases where a slower-footed player wouldn’t.
An update to xwOBA made a few years ago added speed as a consideration on balls that are considered topped or weakly hit, and, as a result, speed doesn’t have as strong of a correlation with xwOBA overperformance and underperformance anymore. Still, though, speed is not a consideration on other balls and fast runners can still steal extra bases on balls hit to the outfield.
Speed matters when considering wOBA vs. xwOBA performace, just not as much as it used to."