An interesting query and one that confronts us almost everyday in ordinary usage too. There is in written english now, a clear distinction between American English spelling versus English (the Queen's version, from the hallowed island) English! The S's are almost always converted into Z's by the Yankies - realised/ realized; the U's are done away with again by the Americans - favour/ favor. the American style obviously follows the way the word is actually pronounced, but the spelling distinctions are documented ably by the Oxford(Brit) and Webster(US)dictionaries. Oh! and whats the word for it? Am sure I dont know! :-)