The UK and the US versions of the book are different in terms of vernacular. Only obvious if you compare them side by side e.g. in UK versions describing Hagrid's hands - big as dustbin lids, US version - big as trashcan lids. The stories are the same but making them coincide with the books might have something to do with the different versions? It's the only thing I can think of.
I have a couple of the Fry books and all the Dale versions, I can't see why they wouldn't just have one of them read both versions either, with the edits for local slang???