It is USUAL for movies to be different than the story told in books. Slumdog Millionaire is no exception.

Often, readers of a book will be grossly disappointed in the movie based on the book read. Thus, it is not unusual to hear, "Oh, the book is so much better than the movie."

Often, the movie writers will add/delete and come up with a screen play that they think will be appealing to audiences that will buy the tickets at the box office.