Yep, as you go to a new tier your average starts again so if you hit 2 rounds, 70 and 80 you'll have 75. Most players go up after reaching a new tier, then you hit the magic number, 40 for master, and only the 40 best scores count. Hit a bad one and it's ignored, hit a good one, it counts and the worst of the 40 is wiped out. My average got very close to 70 after becoming master, now it's dropping again. I like the system a lot, it keeps the challenge going.
I think it's 50 for tour master, don't know about the others.