For those of you who wonder how specific books get on the shelf of your bookstore, I'll explain how it happens.
A seller for the publisher sits down with a regional manager for a bookstore. This 'region' can be as small as a city or as large as several states. The seller hands the buyer a list and says, "This is our young adult line for Fall 2013. We're really excited about X, Y, and Z." The buyer picks a handful of books from that list. The seller hands the buyer a different list and says, "This is our women's fiction line for Fall 2013. We're really exicted about X, Y, and Z." The buyer picks a handful of books from that list.
Sellers quickly learned that they sold more of they gave the buyers a bunch of smaller lists to pick from - dark fantasy, young adult fantasy, horror, epic fantasy, sword and sorcery - instead of just one longer 'fantasy' list.
And that's the origin of most book genres.
Fans then fixate on these categories and attempt to make them meaningful. Science fiction fans are some of the most exclusive types. There's a huge number of sci-fi fans who claim Star Wars isn't science fiction despite it being far closer to the 'science fiction' tales of the 1930s-50s than most hard sci-fi.
