Howdy, Owlblink.
In some of the historical battles we take as themes for scenarios, one side had bombers and the other didn't, or one side had carriers and the other did not. If we want them to have a historical feel, that's how they were. In the case of 8th AF strategic bombing, the whole heart of the battle is US fighters trying to get US bombers to target, and the Luftwaffe fighting to stop them. Even if we wanted to give the German side bombers, the Ju 88 couldn't be it. It would get annihilated compared to B-17's.
Now, figuring out the right balance is less certain than if the sides are symmetrical, but it can be done.
Here is a simplified case just for illustration. Let's say you have 2 P-51's and 2 B-17 formations against 2 109K-4's and one FW 190A. Let's say the scoring is such that a kill is one point and a formation hitting its target is worth 3 points. Now, in the battle, the two K-4's tie up the two 51's in a fight. Meanwhile, the 190 attacks the bombers, shooting down one of the formations while the other one hits its target. The points are 3 for the allies and 3 for the axis for that particular result.
There are more complications in a real scenario setup, but the balance can be there we know because the historical running of them came out close, or one side won some frames, and the other side won other frames, or it was run more than once with the ultimate victor being axis in some runnings and allies in others.
In this case, we are basing the setup on past scenarios that worked out OK, using their plane sets, their relative numbers, etc.