@holon42
It might go as far back as the days when you only had houses, rather than apartments.
So deliveries were either all to the front door on the ground floor or the basement/cellar (coal, firewood and the like) where a house had one. So as houses began being converted to apartments, the terms just stuck? Maybe.
@holon42 Quite possible. Tenaments and poorhouses were very much a thing, of course.
In the big houses, the 'help' used to live at the very top of the building and work at the very bottom. Out of sight, out of mind, unless absolutely necessary. I wouldn't be at all surprised if the terminology is a hangover from those days.
@stueytheround well, thanks, that's a good hypothesis, then. interesting, these cultural choices.
@holon42 Indeed.
@stueytheround yes, that's reasonable. how did the old manor houses label their upper floors? people tended to follow the usage of their "betters" in those days. and they did have tenements in London, no?