<rant>
This is one for the stats nerds, but also one for the reality nerds:
Adam Eaton sucks. Like, objectively, he sucks. He's not a good hitter and he's a crap defender at a corner OF spot. But Baseball Reference suddenly rates him as a very good RF when he goes back to Chicago, the same park where they rated him a very good RF after rating him mediocre in Arizona (in CF, alas) and awful in DC. So, who's wrong: Baseball Reference, or your eyes, who tell you that a craphead outfielder who can't hit .200 and sucks in the field is not worth half a win above replacement?
</rant>
Defensive stats are mostly voodoo. They have some guys who never played baseball sitting in rooms watching tapes and making judgments about who could have gotten to a ball.