Agreed... Inconsistent, too reluctant to book Cambridge for timewasting. I think the ref would have been in trouble if either side had engaged in significant simulation or covert nastiness.
I think it must be quite hard for the players to know what will be deemed 'physical contact', and what is going to be a foul. In the middle of the pitch, this does not matter greatly, but in the penalty area or just outside it, players may get caught out.