To be fair, a lot, but certainly not all of Americanisms are actually relics of the English spoken and written at the time of mass migration to Northern America. Language in many cases evolved more rapidly in Europe. In a related way, quite a bit of the ways in which Mexican Spanish differs from Spanish Spanish sounds very archaic to young Spanish ears, or so I'm told.

I remember seeing a web site where they had the British flag next to "English" and the American next to "English (Simplified)"