MWNL isn't working on the basis of wins this time AFAIK.
It says the percentage is based on the number of additional players recruited to the cause, which I think means the number of additional players playing the game with reference to how many players play that game 'usually'. So for example, if 5000 players play COH2 usually, and during the event 7000 players are playing, COH2's percentage would be 40%.
This puts DoW in the biggest advantage imo, since it's very old and not many people play it. So let's say, only 1000 people play DoW regularly, but during the event 1500 play, DoW's percentage is 50%.
Anyway, this is my interpretation of the line
(*)% of additional player recruited to the cause, above and beyond the standard ranks
and I could be wrong.