I guess it's similar to the way different languages refer to stories, heck in Dutch we have two ways of referring to them:
The word "etage" is used for all stories including the bottom (street level) one, while the word "verdieping" is used from the first story up.
The "2nd etage" is the "1st verdieping", get it?
And it's analogous to indexers in computer languages. Some languages start arrays at index 0, some at 1.
I refer to level 1, 2 and 3 Cities, but don't really know why, just seems more natural to me, but it's more personal than I thought.