It's like Opy said, each time you move up a tier, your average resets. In each Tier, you have to have "X" number of rounds before you reach what we call saturation. For instance, a Tour Master must have 50 ranked rounds played as a Tour Master then their average "saturates". From that point on, your average will not go up. but as you shoot lower scored rounds than the rounds in your 50 rounds, the highest score drops and the lower score is added so your average goes down. At that point, WGT uses your best 50 rounds to determine your average and that is linked to the magical number that allows you to move up in Tier. So as a Tour Master (in the example) when his average has dropped to 61.00, he would then become a Legend and the process starts all over, but only with more rounds required in the saturation.
I hope that makes some sense.
LD