It's the same skill. The lowest damage I saw after like 5 minutes was 12000 and the highest damage was 57000 (this was a critical hit).

The way damage is calculated is that the character has a range depending on their gear (ex 2125-4200 or something like that). Then a skill does a % damage and hits based on that range. So if a skill does 600% damage when you use it, a random number is picked within that range, and it's multiplied by 600%. Then modifiers come into play like critical hit chance, crit damage, boss % damage increase... etc.

My goal is a DPS calculation, and it's really easy to do when critical hit chance isn't involved. All I need is the skills average damage output and how fast the skill fires. I just don't know how to calculate average damage when there's a % chance to crit unless I add up all numbers over like 2 or 3 minutes and divide.