It can be tricky, these days of slipstream and cross-genre fiction, to figure out exactly how to categorize a new piece of fiction. And of course, details of categorization can cause all sorts of kerfluffles.
But even as the question of where to shelve a given book has become arguably less important, there are still reasons to get the proper pigeonhole. On the other hand, HP Lovecraft got along just fine for years while being miscategorized.