Clasifying books

I went to the public library with Izzy today after school to check out a movie. As I was skimming through the children's books, I noticed a category labeled "Fairytales". Knowing how much the kids like fairytales, I started looking through the books. I soon found myself in a new category. I was pumped when I noticed all the books about Jesus and His life. I started thumbing through a couple. As I looked up, I noticed that the section was titled, "Myths." I looked closer at the books around the Jesus books. There was a huge range from Jesus to Al Queda. I got a little weirded out by all of this, but haven't really put my finger on why?
Do all libraries classify books this way? Maybe I haven't been in enough libraries or taken the time to really look or care how the books are sorted. Does anyone have an explanation for this?