I swear on my galoshes this is just a curious question, NOT an obsessive one! (I'm so afraid to seem obsessive these days.)
Have you noticed on Amazon that sometimes the categories of a book will be listed? For example, the overall sales rank might be 242,859; but then there's a little line that says something like this: #81 in Books > Children's Books > Social Situations > Friendship
But some books DON'T have that. How does that come about? Anybody know?