Is it not also because there are also less players with CoH2 + WFA than players with just CoH2?
I Think the disparity hails from the change to AT. If we take this player as an example http://www.coh2.org/ladders/playercard/steamid/76561198012811763. He has randomratings with the old factions but not with wfa. Probly he hasnt played a singel game as a randomteam but the decay in the ladders is so slow he will be there a couple of years more. So the randomrating works nice for wfa but relic could reset the old factions random-ladders.