... is differential understanding and application of the rules.
Lets face it, some players call pick if they even have to think about any other body on the field besides their mark - some only call it when there is contact between players and have no problem running through the stack to follow their guy so long as they don't have to clobber anyone. Some call a travel on the fast breaking give-goer, some don't. There is a wind range of violation criteria in our players' minds. Further, call selection tends to change as the game gets closer to the end.
Compare this with soccer officiating. I have been a referee for 11 years. Yes, different referees call things differently. There even is a change in call selection when the ball is in the penalty area and depending on requirements of game control. One referee will even call a foul in one game that he let go in a previous game. The difference, is that all the decisions are left to one mind who applies the rules equally to all players and both teams. Even if one team completely disagrees with the foul selection, at least both teams know the standard.
With 14 different referees on the field, it is very hard to have a consistent standard for what is deserving of a call and what is not.