There are two types of library classifications, the Dewey Decimal and the Library of Congress. According to an article on the University of Mississippi website, Whether LC or Dewey is used in a library is based on the size of the collection. Dewey is divided into 10 classes, making it better suited for libraries with smaller collections, such as school or public libraries. LC has 21 classes, which allows for more call numbers needed for the greater number of books owned by larger academic libraries.
Intrigued by the history of these two classifications? I was, and found this pretty great article from The Straight Dope.
And now, finally, thanks to newhousemaps the creator of this infographic, the numbers make SENSE!