Help to understand "mirror" game setup

[MGK(FGM)]

For me "mirror" games are good way to have balanced ladder result in unbalanced periods in CM. It is also possible to play 2 games and add "small" points from both games and then register it as one 2 times bigger game.
With Ramagel we are playing just for fun so our "mirror" games are Romanian armour attack against Soviet infantry(no rarity) in 1943 and almost all rest is set to random :) including AI purchase :) so it is broken mirror game :) but should be fun