Just did some testing on it, and, for instance, a rank 1 rain of ice did between 190 to 210 damage, every single time. Rank 2 did 380 to 420.
This looks to be just a little bit of randomness that I'm guessing is present in a lot of effects that we simply were aware of. I have no idea why this is done, but there it is.
Would be nice for someone at GPG to confirm mystical gameplay mechanics like this.