The new average system started about a year ago. Before that, we had a (more or less) true average. Then, it was the average of our 10 best rounds out of the last 14. Now, we move up a little when we score well and nothing seems to happen when we have a poor round (The algorithm has never been made public). Keep in mind that only rated rounds affect our average. More info on this in the FAQ Home over there---->
Joey