Once you reach any given tier - in your case, the Master tier - you need a minimum number of ranked rounds before you can progress to the next tier. I believe WGT calls this minimum number of rounds the 'saturation point.' If my understanding is correct, prior to reaching this saturation point, all stroke play ranked rounds on any Par 70 or Par 72 course will be used to compute your average. This means that any rounds above your current average will carry more weight against you, so to speak. But once you reach your saturation point, then only your best rounds will be used when calculating your average. This means that the reward for a round under par is much greater, and punishment for failing to break par, or shooting above your average, is negligible.
But I've only been on tour for a couple of months and I'm no expert in the matter, so everything I just said take it with a grain of salt. I'm sure someone more knowledgeable can give you a better explanation. Good Luck reaching Tour Master.