[Avistew]

Well she wrote the books in British English and, if I remember from her interviews, then sent it to her editor for the American version, who would go over it and change it to sound more American. So so me it's more of a... localised version, I guess?
Which I find silly since it takes place in Britain, it seems more normal that they'd speak British anyways. And it's not like it's going to hurt American readers to learn British words and expressions.
Considering the British version is also the one that was sold in Canada and France (the only other two countries that I've checked) it seems to me that it's the "real" version, and the American one is an adapted version, not quite a translation but not quite the original either.

EDIT: Thinking about it, I think a bit part of it is how the first three books were released in British English only, then "converted" and released in the US months later, as a translation. Then it's true that they were released at the same time once the series was really famous, but I'd guess that it was just the British release being delayed until after the American version was ready.