Apparently, the damage is calculated the same way, it's just that when repeatedly trying to win the same battle, for example, in a mini-Naga bank, where a stack of skeletons should deal 220 damage with a range of 65-250 (roughly), to kill 2 Nagas in one hit, it's not that easy to achieve.
By the way, doesn't the fact that repeating actions in battles results in the same battle outcome also play a role here? I'm not sure about the damage, but I remember that when attacking a high-level dungeon and getting morale from the guards in the first round, in replays, this creature also got morale in the first round. Although, if my memory serves me correctly, this is a feature of the HD mod.
It seems like it would be good to add a plugin to H3DamageCalculator that calculates the probability of dealing/not dealing damage greater/less than a certain damage with a stack. :D