Omalla kokemuksella voisin kyllä sanoa että sellaista kuin amerikan englanti ei ole olemassakaan.
Kyllä se vaan on olemassa. Eroaa tuosta toisesta sanaston osalta jne. pikkuseikkoja. Tässä sitä on ihan hyvin selvennetty: http://en.wikipedia.org/wiki/American_English
"American English and British English (BrE) differ at the levels of phonology, phonetics, vocabulary, and, to a lesser extent, grammar and orthography. The first large American dictionary, An American Dictionary of the English Language, was written by Noah Webster in 1828; Webster intended to show that the United States, which was a relatively new country at the time, spoke a different dialect from that of Britain."