EASY answer, when MANY scoring drives are stalled due to penalty, YOUR "D" don't need much to improve. What I'm saying is when the opposing team is cut short 30% of it's oppertunity to have a scoring drive that one of the benefits is YOUR defense looks a hell of a lot better.
I give you the TITAN's game as an example and proof.
Salute the nation
As much as you don't want to admit that, it very well could be true with the Cheats history and especially how Goddell has mostly let them off the hook with slaps on the wrist. I do find it interesting that they can almost instantly go from giving up north of 30 to shutting teams down. You can't just flip a switch like that.