In my experience it has little to do with the airline but more the ground handling agent. Our local agent has a monthly target to meet and towards the end of the month if they are down then the supervisor will patrol the check-ins jumping at every opportunity of an excess charge. Luck of the draw I'm afraid but you do say that you've got away with it in the past so look on this as a balancing process. Much like parking/speeding tickets really... some you get and others pass you by but you usually finish on top.