If you remember back to last season, it became an article of faith that Mike Trout was the finest defensive outfielder on the planet. It was one point on which almost everyone could agree. Seamheads could say, "Well, just watch him play" and, sure enough, he'd cover an ocean out there. Meanwhile statheads could trot out that he was worth +21 runs on defense and a 2.1 dWAR, which became the linchpin of the MVP argument for Trout.
Yet this year Trout is worth -12 runs (at least according to Baseball Info Systems) and has a -1.1 dWAR. He's almost a dstat twin with Shin-Soo Choo. Suddenly Carlos Gomez is the new gold standard in the outfield.
So what changed?
We'll probably see Russian novel-length breakdowns trying to answer this question, but let me cut straight to the answer - nothing. Nothing has changed. Mike Trout is every bit as fast as he was last season and possesses all the defensive capabilities he did a year ago. He just isn't getting a randomly generated hit array around him that allows him to post the same gaudy numbers.
BTW, Gomez has been the gold standard in CF for pretty much the past four seasons. It's just that this season he's got gaudy defensive numbers.
Anyway, I think the lesson with Trout is that we're missing something incredibly important when it comes to quantifying defense, particularly in CF where the defensive talent is outstanding. We're not able to separate circumstance from ability. No one should put much stock in circumstance. It comes. It goes. Numerically, ability remains elusive.