all dc/dmc players perform better in dmc position.

It is not up to the player, it is due to the position itself.
DMC is involved in many aspects of the game, marking, passing, shooting, so his rating can easily be great.
The only time I see dmc completely failing having a bad rating is when he is outclassed by an amc..

On the other hand DCs (when they do not score) they have low rating, you may win, all your team be 7,5+ but yet DCs can stick around 6,5.