I think in most cases, it's used to describe the same 48x18x20" tank. But some manufacturers make a genuine 80 gallon that is 48x15x31 (or something comparable).
i am looking at the standard one (first listed).
i have seen it stated that the footprint was the same and i can't imagine that the 75g would be any shallower than the 80g. but then i suppose it would only be an inch or so.
it's really a cheap tank for the gallonage and it seems less popular than the others up or down on the size scale. i wonder why.