Defensive efficiency is just 1-BABIP, so I guess it is really a question of whether defensive efficiency/BABIP is something that should be attributed primarily to a defense, to a pitcher, or perhaps to dumb luck. The rest of the NL Central didn't fare as well as the Cubs, so at least anecdotally it doesn't appear to be attributable to the quality of opponents faced. It's probably some sort of a combination of defense, repeatable pitching skills and luck; I just hope it turns out in this case to be more from the first 2 categories, and less due to luck.