Think there only charging for bags that are oversized! too often passengers who only have hand luggage, check in online, and when they come out too the aircraft their bag is too big for the cabin, and it has too be off loaded and placed in the hold! therefore avoiding the baggage charge! so now if your bag is too big and doesnt fit the requirements (sizes) the ground staff charge a fee and give you like a fine for the bag! and the cabin crew take the payment from you onboard. i think its a good idea, everybody knows the required size of bag your allowed but still try there luck and bring bigger!
