I don't know enough about American English to "explain", unfortunately. I just notice that American speakers sometimes use one form when the other would be required in British English. For all I know, from an American point of view, perhaps British speakers are failing to make a distinction. However, something seems to have happened to those verb forms, and I think the change must have happened in colloquial American English because in this particular respect it's colloquial American that is different, while formal American, formal British and informal British all seem to agree with each other, as far as I can tell.