Random magnitude damage over a duration is indeed random
Damage spells in vanilla only deal actual random damage values if they're instant. Over longer durations it seems the damage is fractionally recalculated every frame (or at least more often than every second), which effectively averages the total damage.
I think it's a bug with the original engine and not desirable behaviour, but YMMV wrt the effect on what passes for balance in the vanilla game. For instance a "god's fire" spell would consistently damage the player for around 350 in vanilla, but in openMW it may graze the player at 120 damage or one-shot (over 10 seconds) at 580 damage.
(RM-4297 from redmine: created on 2018-01-11 by rot tor, )