On the interception they were bodied up and frankly it's just not obvious. There isn't a ref to look at every match-up, so just like with linemen they are going to call the obvious things. A jersey stretched 2' is just far more obvious than when two guys are shoulder to shoulder and one of them grabs the other.

A big part of the game is knowing what you will and won't get away with. Grabbing a guy from behind by the jersey is just way too obvious to not see.