%Proof was always a variable quantity. IIRC "proof" spirit was originally that which, if poured on gunpowder would still allow the powder to ignite. Thus any alcohol (by vol.) content between 73-100 would be "proof spirit". Anything below would be shown a %proof based on the lower limit. (i.e. what quantity is left after removing water by further distillation thus bringing the spirit up to "proof".)

Nigel