Magnifying glasses do not always make matters clearer. There has been much talk about American English in terms ancestral. Researchers have analyzed speech sounds and “derived” them with particularity suggestive of Pygmalion:
I have never pondered over any possibility to become my grandfather. Anyway, my grandfathers as well as grandmothers did not speak American, or actually any English, as far as I remember. My father spoke some English, but he had an accent and told me to pick up on my own, I was little enough to do that. If I wanted a grammar book, he would buy it for me, same for note books and other stationery, but he would not teach me. He was right, though he was a historian.
Back to deriving American:
“The main idea of the approach is that the origins of American English are somehow contained in the various regional dialects of British English…”
American English, an Introduction, by Zoltan Kovecses.
Ben Trawick-Smith makes an interesting point: we might think about the British as “talking American”, as well. He includes American English with “a larger continuum of Southern England-derived dialects”. He yet adds the idea is debatable. When Did Americans Stop “Talking British”?
What American English would the talk be about? If we do not say, the American English of the 1900s, or 1800s, we say contemporary American English.
The present-day form of the language originated in the USA. Part the speech sounds, isolated, might resemble British. It is yet inevitable. We could not want a language without speech sounds, to have a language of its own. However, we always tell origins of languages by lands of emergence.