It is the same weight as if the bag was checked
True, but if the bag is checked the weight is know to be there and will be used for weight & balance and performance calculations. People who wear their baggage on them and pretend it's just their jacket will appear on the loadsheet as a pax at standard weight. The weight of their 'baggage-jacket', as it is unknown, will not be used for either W&B or performance calculations but it will of course affect both.