i am confused

As an American, I find that it can take a few minutes of actively listening to someone speaking in another accent to sort of fine-tune my ear, but once I get used to the changes in intonation and pronunciation of vowels, it's pretty smooth sailing, since the words and the grammar are the same. Short answers can trip people up, though -- I remember a waitress in England asking me what I wanted to drink and then looking at me blankly when I said "water". That's because she was expecting "wo-ta" and I say something that sounds more like "wadder". I did get my drink, though, so it worked out in the end.

<<over the centeries, U.S. English has trailed of a little>>

<<I thought British English trailed off more.>>

Neither one has trailed off while the other one has stayed put. Both 21st century US English and 21st century British English share a common ancestor which they've both evolved in their own ways from.