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The damage range for each unit in a stack is calculated separately. Therefore, it is perfectly natural that the more units there are in a stack, the lower the probability that the entire stack will deal damage close to the minimum or maximum. That's the math.
Specifically, if you have one unit in a stack with damage, say, 2-5, then the probability of dealing 2, 3, 4, or 5 damage will be the same. But if there are 100 such units, then the probability of dealing 200 or 500 damage will be much lower than 350.
UPD: I read that the author is aware of this. Then, in my opinion, there is no need to change this, even if it is possible. It is perfectly logical that the more units there are in a stack, the smaller the damage range should be. If the range is made equally likely, it will be unnatural from a mathematical and logical point of view – a clear negative for the gameplay in the end.