Mighty Gorgons

You are absolutely right, if you have 10 Gorgons, there is a 100% chance that they will kill one monster from the enemy stack, but more than one might die; everything is within a range. The probability that 1 to 10 will die is 1-100%, and the probability that all 10 will die is 0.00000000001 (you need to multiply the probabilities, 0.1 raised to the power of 10). So, we have a range from 1 to 10 with a probability event in percentage terms.

From each group of ten, one unit can die.
1-10 units - maximum 1 unit
11-20 - maximum 2 units
21-30 - 3 units, and so on.
It is difficult to say with what probability this will happen. You can rely on the data provided above with the formula, but I think they are not correct. Here, the probability is not for the entire quantity as a whole, but somehow calculated for each group of ten separately; otherwise, I cannot explain it. It is better not to dwell on this question.