well on the knowlededge that some players are easier to maintain GS than others, for various factors - they are probably just leveraging that same principle.
might be a standard factor thats applied across the existing player GS variability.
so by that logic, if you were to assume that all players who are currently at mid-level respectable GS, if a guy drops GS during player transfer, then you could make the assumption that the ability for him to maintain GS in general, will be difficult.