Create a game of 3 players and an AI. Have 3 players joined.
Edit the game and reduce from 4 slots to 3.
Failed Method
Reduce number of slots to 3. This chops off a player and not the AI. You cannot remove the AI this way.
Successful Method
Increase the number of humans to 4, then reduce the number of slots.
This could explain why many games by new players have an AI slot at the end.