That’s correct. "game_id": "" is filtered out of the results of top games, but is done so after the results are returned from the database/cache, so if that game id is in the results range requested, it’ll get dropped and you’ll end up with first - 1 results.
This can be verified by using https://api.twitch.tv/helix/streams?game_id=0, paging through the results and summing the viewer count. The position where that game would be, if it was included, is consistent with where you start seeing a results returning fewer than the first param.