@Starson: Exactly
Add to that, that I am German and like to read both German and English language books, which makes metadata fetching an even more difficult excercise.
@spacemonkey: Afaik the isbndb API uses the same algorithm and sorting as their website. At least that's what the API documentation says. I think some of less than optimal results in calibre comes from the merging and resorting inside calibre.