Could be he does a lot of deception with the offense and the opposing team scouting gets caught up.
Could be the team has been so effective passing the ball and it is harder to throw in December.
Could be the team has had injury bug hit (that is one of the most interesting part of NFL coaching to me is managing the medical part, the medical staff, the bodies, the treatments, how much to work, how much to rest, the physical therapy, Massage, when to sit with injury, when to play through it, etc)
It is something that I wonder if one team does better than another team, how much of a difference it makes. Maybe, they can hire Lebron as a consultant because he seems to have it down.