Thanks for pointing this out, SailorYue.
The last episode was explicitly stated as its 100th episode. I think the problem is the way that the movies were added. Comedy Central aired each movie as four episodes, and each episode had its own production code at the end. Someone consolidated them into movies instead of listing them as separate episodes.
I'd fix it, but the problem is, I'm not sure where they should properly be. They weren't really a part of season 6, but they weren't season 5 either.