In the David case, the coaches probably also saw that Davis can do amazing things with the ball... and that he makes mistakes in the other tasks of a RB. Brooks, being a grad transfer, probably is/was better at all the other things.
BUT, with wear and tear, the other RBs drop off a bit, and Davis learns more of the things he has to do, and eventually that decision switches.
I think the same is probably true in the Defense as well.
We talk about "Fresh Legs" in the 4th quarter of the game ... I think there also a "fresh legs" element of 4 games into the year as well.