As lick touched on, the average you see is not a lifetime average. For all intents and purposes it is a tier based average, used to calculate when you should move up in tiere. Every time you move up in tier the average resets and all scores from that point affect the new average. So whatever you shoot on that first ranked round after you tier up, is your new average.
Additionally every tier has a saturation point, whereas, bad scores will no longer affect your average and only good scores will bring them down. I can't remember what those saturation numbers are exactly, but they are something like:
Hack 5 ranked rounds
Amateur 10 ranked rounds
Pro 20 Ranked rounds
Tour Pro 30 Ranked Rounds
Master 40 Ranked Rounds
Tour Master 50 Ranked Rounds
Legend 500 Ranked Rounds
Tour Legend 500 Ranked Rounds
If you want a true WGT lifetime average you would have to keep track of all of your scores seperately. Additionally, Best of Par 3 and Par 5 don't affect your average or your Ranked Round count in that given tier