Perhaps they're on commission. When I worked at MyTravel, the agents there were on commission and some were pretty tight with regards to charging for literally anything over. Especially around this time of year, to help boost their pay packets for the party season. All airlines are coming down hard on excess baggage now and unfortunately if you're over even by a fraction, expect to pay. Really just depends who you deal with on the day. Some agents will charge whereas others will be more lenient, perhaps the check in agent was having a bad day and decided to take it out on you, maybe there was a supervisor or manager present at the time you checked in. There could be various possibilities. At another time, on a different flight, with a different agent you may well have got away with it. On this occasion you were probably just unlucky but watch out, coz most airlines now will sting you for every kilo over if they get the opportunity.