Tons of factors. Age, bench players, injuries, etc... Also worth noting is that Defensive Efficiency is simply the reverse of batting average on balls in play. It just keeps track of whether or not the play was made... not whether it should have been made. So it is possible the opposing teams had a disproportionate number of times where they simply managed to "hit em where they aint," so to speak. I would caution against chalking the whole thing up to bad luck though, we've definitely gotten a bit worse in the field the last few years. UZR would take into account the factors listed in your first line. It would also taken into account whether we've gotten worse in the field from '09 to '10. I'm aware. I was just responding to (what I interpreted to be) mojo asking how our defensive efficiency could have tanked so quickly. Now that I'm rereading it, it seems he's wondering about the discrepancy between UZR and DE. Both, actually. I'm genuinely curious.