The best answer I can give you is ... maybe.
Repeatedly, when an 80/20 possession is encountered and the formation is otherwise legal, there's a player of another position playing in the GK spot.
BUT, testing an otherwise legal formation using a non-keeper in the GK spot, does NOT result in 20/80 possession.
I have no idea how to explain these results. So far, nobody else has come up with a working theory either.