Average? But what if someone only plays correspondence? Then ratings for other types of games won’t be changing at all. For example, I don’t play live at all so live ratings are 3 stones lower than correspondence.
So as I understand, overall rating calculates separately based on all games you play.
Overall rating may be higher than others, if player is improving really fast, I think. I found good explanation here: How is the new "overall" rating determined?