Sounds to me like they are looking for more solid sign ups and by saying that the commissions will vary dependent upon how solid the sign up is, is meant to drive affiliates to target and screen better for the people they refer.
It may be a move meant to deter the thought of posting willy nilly by affiliates on such as messenger channels and forums and encourage safe list building practices to pitch the program to.
I believe they are also wanting to see affiliates bring a steady stream of referral traffic versus just driving traffic here and there when they have the time or feel like it. They want to see that the affiliate is producing consistently thus increasing the reward for such effort.
I may be totally off base but that is how I am reading into what they are saying at least in part.