Efficiency of performance, among other things. It tries to mitigate the boon or bane of a close win or loss too, so a close loss isn't seen as too much different than a close win. (So for instance OU gets dinged for letting a terrible Baylor team hang around for awhile). It's a crazy amount of data that spits out some real oddballs at times, but can also be a good indicator of overall team quality and performance.