One of the quirks of travel. Cabin baggage goes by the size of the bags, not the weight and when a hold allowance is 20+ kilos, a litre of booze or so for duty free (which you can only buy so much of) won`t make a huge difference. The passenger could be 20 K overweight so it`s all much of a muchness. The overhead lockers do have a combined weight for all the bags in there so where some are heavier, some are lighter so it all adds up to the same. It`s the size of the airside shopping bags that are the problem in which case people will be told they have to go into the hold