This is usually due to RAT not seeing them log out. Meaning, either RAT wasn't running when they logged out, RAT missed the event for a reason (a mod changing the message, or... just missed the messed), etc. So they are still listed as online.
When you start RAT, I set all players to offline and then requery the player list and update then. That way things keep in sync at some point. However, the bigger reason is why does it keep happening. So, check for any mods that might alter a disconnect message, errors that get thrown when a player disconnects, etc.