Well, it’s purely a convention.  But since F4map and other 3D renderers go like that, we should follow it indeed.

It makes sense in this simple wiki example: https://wiki.openstreetmap.org/wiki/File:Minlevel.svg
but does not work well with piled structures.  (And IMHO it would make much more sense to map coherent group of floors as polygons instead of slicing floors.)  Sorry about the previous edit, it should be correct now.