CTTH competitions are not factored into the average that you are asking about. Your average is only affected by 9 or 18 hole stroke play ranked rounds (including any and all tournaments and multiplayer games)
So you know, the average defaults to 150 when you sign up and is determined by the best 10 out of your last 14 rounds recorded with heavier weighting to the last 3ish rounds. So while it seems that the Tahoe challenge is affecting your rating, it is not. You just need to get more ranked rounds in!
Have Fun!