I'm sure we discussed this is the long distant past and someone found a reference that in fact Europe (including the UK) used to use the 1,2,3 system and changed to "ground floor" at some point, whereas the USA (and presumably Canada, didn't).

Edit: Here it is -
From Lucy:
My rather elderly Shorter Oxford gives the following:
First-floor. 1663.
1. The floor next above the ground floor 1865.
2. The ground floor. Now only U.S.

http://wordsmith.org/board/showthreaded.pl?Cat=&Board=words&Number=2056