It's extremely difficult to predict what specific players will do in the future.
In general, these projection systems are excellent. When it comes to certain specific individual players, however, they can be super wrong.
With Bourjos, I'm guessing that Steamer just doesn't give him as much credit for fielding as he's earned while playing in his 350 some odd games so far. So if he's played like a +20 run centre fielder, Steamer probably takes a more cautious view of him and pretends that he's a +8 or something like that.