Until I see stats I won't wager a guess.
I remember about a dozen years ago a guy went through and broke down all the team penalties. New England was toward the middle, as was a few other teams. New Orleans, the giants, and some more, right in the middle to high side.
The guy then broke down yards per penalty and there were outliers among the teams that eventually showed bias but they still ended up as getting fewer yards then average.
But, the stat he provided that broke it down essentially put it as penalties which has significance to the outcome and the effect of the game. It was magnificent. How many penalties came in each quarter was there. How many penalties when the team had a greater than 10+ lead. Etc
It clearly showed favoritism. The guy wound it up saying that he believed it just to be a fact that refs believed these teams were better than the others and thus. Less likely to have penalties committed.