I have, like everyone else, been wondering about this for weeks now. The only thing I can think of is that it is much more difficult for the refs to detect holding in the middle of the line than it is to detect it against rushers coming from the outside. I say that because if there was a defensive end who was held as much as Donald I think the penalty would be called more often than not and the opposing team would be forced to make some kind of adjustment such as using a TE to help out. Even on television it is much easier for me to see holding against the guy coming off the edge. Just wondering if opposing O-Lines know how to hide holding in the middle and if the refs position at ground level makes it hard to see.