A few things to mention so I can get a clearer picture here:

1. Each page is cached and it's possible that new data has been updated since the time of the original cache leading to either wrong counts and/or additional pages
2. In those cases when you're checking the second page, are there results? Technically you can request any page you want, but that doesn't necessarily mean there will be results

when i check the second page, there are always results, and the results length is 20. but the first page is not. for example:

  http://api.themoviedb.org/3/discover/movie?page=1&with_genres=28

returns only 19 results, but

 http://api.themoviedb.org/3/discover/movie?page=2&with_genres=28

returns 20 results

http://api.themoviedb.org/3/discover/movie?page=1

returns only 9 results, but

http://api.themoviedb.org/3/discover/movie?page=2

returns 20 results.

Ya, this is a combination of a few things that I'm aware. I've created a new ticket to look at this in the future.

Easy fix for you should be to trust the total_pages field, ignore the number of items being returned in results array.

thanks for your quick reply. much appreciate your help ^{^}