xFIP stands for Expected Fielding Independent Pitching, which is defined like this:
Expected Fielding Independent Pitching (xFIP) is a regressed version of FIP, developed by Dave Studeman from The Hardball Times. It’s calculated exactly the same as FIP, except it replaces a pitcher’s homerun rate with the league-average rate (10.6% HR/FB) since pitcher homerun rates have been shown to be very unstable over time. A pitcher may allow a homeruns on 12% of their flyballs one year, yet then turn around and only allow 7% the next year. Homerun rates can be very difficult to predict, so xFIP attempts to correct for that. Along with FIP, xFIP is one of the best metrics at predicting a pitcher’s future performance.
Some people don't like xFIP as a measure of a pitcher's performance, and some people don't like xFIP as a measure of a pitcher's future performance.
To each his own, but I know some smart people who do pay attention to the metric.
Why do I bring this up?
Kennedy went 21-4 this season, and Gallardo went 17-10.
Kennedy posted a 2.88 ERA, Gallardo 3.52.
Kennedy's advantage shrinks when you consider FIP -- 3.22 for him, 3.50 for Gallardo -- but it's still an advantage.
Ah, but xFIP?
It flips everything around. Because of home runs and the supposed variability therein, Gallardo's 3.19 trumps Kennedy's 3.50.
Oh, and one more thing ... Care to guess who led the National League in xFIP this season?
Nope, not a Phillie. Nor a Giant.
Maybe none of this matters. But there's that thing about future performance...