In every battle there is an X-factor that means an attack might do from zero to 100 percent of the listed damage. This is in addition to the effects of terrain, forts, and home defense.
https://forum.callofwar.com/index.php?thread/6446-the-x-factor/&postID=58160#post58160
There is a limit to how many units of each type are most effective per group, called the State Based Damage Efficiency. Forum thread on SBDE