This is the closest explanation I can find: WGTadmin:
We use an algorithm to determine when a player is eligible to move to a new tier, which takes into account a number of factors including number of ranked games played. These numbers scale by tier, so it's necessary to shoot a lower average to advance and to play more games to reach higher tiers. For example, when you reach Tour Master, you will need to play a larger number of ranked rounds than you had as a Master, maintaining a low average, to tier up to Legend.
Additionally, in response to WGT player feedback about sandbagging (misrepresenting one's true ability to gain advantage by playing at a lower tier from closer tee boxes), WGT is using more than just ranked stroke play scores to determine each player's average score and appropriate tier ranking.
For obvious reasons, WGT is not sharing exact details of these changes, but they include factoring in scores from players who perform well in tournaments and challenge games for credits. We still encourage the community's feedback on Sandbagging, but believe this is an important step to reducing the practice and keeping the game fair.
Finally, most of our top players have posted close estimates for players to try to gauge their progress in advancing tiers. Here is a good thread to read:
http://www.wgt.com/forums/p/9085/707139.aspx#707139