Can someone please ELI5 Firestorm strike and other variable damage specials?

In my experience over few months of using Fjalar, I have NEVER witnessed him dealing the maximum (1080) amount of damage while attacking. In fact, he most likely always does the minimum amount of damage, either in 4 tournaments or against stronger 5 teams. Conversely, when I face such heroes, they seem to always deal the maximum damage.

Similarly, when using Loki (4/90 and 25 emblems) to copy Regin or Hreidmarr special, I always do the minimum amount of damage, but they still don’t have a problem doing max damage on their own.

So what’s the catch? Are these heroes not tailored to attack? Is something wrong in the damage algorithm?