Gas bottles are rarely consistent in their manufacture, so Calor and most of the professional set ups actually weigh each empty bottle and stamp the empty weight on the bottle TARE WEIGHT.
Flipping 'eck. You learn something every day. I assumed that the 7.5kg bottle weighed 7.5kg!! In my defence I've never had to change one. Thats what OH is for. That and emptying the toilet. Dogs...barking yourself and so forth.