I think the difference is that those "Amrican" accents would be meant to sound weird, being parodies, whereas the impression is that the Hollywood versions of the English were intended to sound English. That applied even more so to the quite extraordinary "Irish" accents you used to get in the movies. (And of course English or Irish actors in Hollywood movies at one time used to play along with it, and provide accents that they would never have used at home.)

I think these days more effort is to get those kind of things accurate. A pity really, in some ways. It used to add to the fun of moviegoing.