ORIGINAL: FNG
The net result is that the better quality troops get even better more quickly, because of the number of enemy units that they destroy.
Admitedly, this has always rather been the case with long campaign core units. I've yet to play any long campaign (particularily as German) where a good number of my core units didn't quickly become Elite. Part of the reason are the ones you've laid down....another big factor is human vs AI. The Human will usually win unless he's facing overwhelming odds or is playing a side with low exp and inferior equipment.
IIRC (its been a while since i've done long campaign) i think another important factor is that infantry units dont lose any exp when they replace losses. They just keep getting better and better. Same thing with tank crews....unless totally destroyed you can rebuild them and their exp rating will remain intact.