The addon doesn't display the same rating as the xpoff ladders. The ladders on the site use skirmish data from blizzards own API to determine your rating. There is no addon that displays that rating ingame.
The addon calculates an artificial rating based on your win/loss and is completely unable to take the opposing teams mmr into account.
You can manually refresh your character on the xpoff ladder if you don't want to wait for the update. You just need to log out first and then press the refresh button.