I think part of their progress as a team was the departure of Peterson. As long as Peterson was on the roster, that position was locked, with less open competition. Their gameplans would also be hampered by AP - He was an all-time great runner, but I have always thought he was a problem on third/passing downs. I think it is similar to when we have Shady in and he is forced to provide pass-pro. I'm not sure AP was poor at that, but he always seemed a little disjointed in the pass game.
With him out, they were able to be less predictable.