When you combine several movies in a collection, the counter counts the collection as one movie.
( « Hide items which are in collections » option selected )
–> Your library contains 5 movies: Back to the Future, Back to the Future Part II, Back to the Future part III, Pulp Fiction and Fargo.
You combine the Back to the Future movies in one collection. The counter now indicates that you have a total of 3 movies in your library.