Google search results

Why is it when you search for something on Google it says you get a really high number of results (in the millions for a lot of things), but it doesn't actually give you that many?

If you go to the last page of results it's a lot less than it initially said. For instance, if you search for "google" itself on Google you get 460 results (if you include omitted ones for more), which is 46 pages, but it said there were 13 billion.