Its the one variable no one really seems to have a handle on when it comes to tier progression, credits earned.
Generally, WGT follows somewhere pretty close to the guidelines that Apex laid out, however, from TM and below, if you are placing high in RGs and earning significant credits in them, as well as the weeklies and monthlies, WGT will bump you up before you hit those guidelines.
I started with RGs at Pro. I got bumped up every tier, until legend, well before i met the ranked rounds stated, as well as average. The one major tier that the guidelines held, was for Tour Legend. I was well over 500 RR and i had to get to a 60 average and I was playing a lot of RGs at that point.
I think that WGT uses that one variable as a anti sandbagging, anti multi account measure. If you are a TL or Legend and want to play in the lower tiers with a second account trying to cherry pick credits, you won't be able to spend much time there doin it, which is fair enough. It isn't right for someone who has been playing the game for 2+ years and has everything dialed in, to be playing in RGs and tourneys with people who only been in the game for a few months.
I personally think it is a money/credit measure as well. The jump from TM to Legend has the biggest impact on the equipment that you use. You see post after post here saying that you don't need the top top equipment at pro, TP or Master/TM, however, with the length added at legend, you better have a decent driver(282+) and some good irons(speedbladez or 85 nikes) to compete. If you still got your 59 G20's or something similar, you will be hard pressed to rank in any tourney or RG. So if you are at master or TM cleaning up, WGT is just gonna bump you and force you to use up some of those creds to upgrade your kit. Also fair enough, if you are in this game for the long haul and legit, you won't have a problem with that. You just need to hope that you played the leveling game correct and you are at a high enough level to get some decent gear to compete at legend.
If you are cherry picking it won't get you very far. Maybe enough to fund the next account and do it all over again. Prob decent to good legends that don't have what it takes to make the jump to TL, so they will just spend eternity as Gr8 Pros - Tour Masters and nothing more, or TL's who can't handle the high level of play when you get there so again they restart, so they can feel like a big fish again.
Anyway, I have yet to see a post where someone has figured out how many credits you have to earn or how high you need to be placing at each tier to get WGT to bump you early. It does happen though. I still think matchplay and MPC triggers early bumps as well.