There are about 1000 books which start in the top segment of the display when no filters have been applied. In this example three of the category gates have been opened so books from those categories can pass through. The ones that don't pass this filter pile up near their closed gate which helps give some understanding of their distribution. The books that pass the first criteria encounter a second filter on the average rating of the book from Google Book reviews. This filter gate is set to only allow books having an average rating of at least 4.0 to pass through. The final gate does a pattern match on Author name and allows 4 books to the bottom which have passed all of the criteria.
Google Play Books review - PC Advisor