I am a very weak player chasing, and while I can't put my hands on the joystick I bought to play DF in Windows 8, using a big screen, I will always suck in head-ons as well. My K/D ratio of "zero point something" talks by itself.
All that said to state that I am not ashamed by telling you that I NEVER PLAY DEFEND THE ZEP, as I NEVER PLAY DOGFIGHT! missions. Only bombing missions, kill the zep and CTC.
And because of this, I believe that, as me, many other players feels better playing german planes. When I want to look for a destroy-the-zep mission, and when the server insists on offering me an allied plane, I hit the "buy credits" button, next I cancel it, to quickly return to the main screen and hit the "multiplayer" button again, for another chance to be presented with a german plane. No need to quit and restart the game over and over.
This "preference" for german planes can explain some of the lopsided games (although I noticed many lopsided games when I am the one against seven allied planes, but I suspect that, playing german, you have more probabilities of being part of the more populated team).
Concluding: to make things more balanced in zep missions, one possible solution to this unbalancement on zep games would be the obvious one: two zeps - of coure, one german heading south while the allied one goes north. Duh... And hey, it would involve a lot of tactics. What about it, Zup?