This is not a problem with Hexxit, but with minecraft in general as far as I know.
This is one of the glitches I've mentioned earlier of which all admins always claim to be completely oblivious or that the user in question is lieing about it to get free experience.
Unfortunately it has to do with chunks somehow storing the player's stats and not updating that information untill the player goes through that area again.
Normally, chunks unload after a while, but chunks around a portal will stay in their active state and always consider the player to have the level they have when they entered/left, thus giving you potentially infinite experience if you plan it right.
That being said, I have yet to test if this works in single player(/without multiverse)