In British English the ground floor is the one you gain access to from the street or garden and the first floor is up the first flight of stairs and you go on counting every flight of stairs to name the subsequent floors.
American English calls the ground floor the first floor, hence the confusion.
I assumed the use of "floor" for "ground" was a matter of dialect. I am probably wrong, but the first many times I heard it, it was said by people from Birmingham.
And the reason we discuss these sort of things and permit ourselves to feel some usages are wrong is that we care for language and feel that it facilitates understanding if there is some standard form of a language in use.