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.