Does it matter though?
If you generate one 0.8-1.2 value for all classes, you'll get the same average damage as if you generated a separate value for each class. It would be game-changing if you had like a stack of Heavy Tanks and artillery, and you attacked it just to kill the artillery (knowing that the heavy tanks will kill you afterwards), but actually it still doesn't matter: if the value is generated against the artillery separately, your chances are 0.8-1.2, if it was generated all-round, your chances are still 0.8-1.2 (the fact that it will also apply to the Heavy Tanks is irrelevant, they won't do different dmg back to you because of it)
So all in all its just irrelevant. To be fair I have no idea about this, I assume though that the value is generated for the whole army and its damage values are multiplied by it
You can test this conveniently with 1 heavy tank + 1 AT gun armies fighting each other...