Fatigue, Gem Use, and Randomness?

[tinkthank]

I have watched a couple of battle replays to make sure, and I dont see how this is happening:

- An Air-5 mage casting Wrathful Skies with 3 gems (he had 3 in possession, used all) with 8 spell-casting encumbrance ended up with 61 fatigue.
- An Air-5 mage casting Wrathful Skies with 2 gems with 8 spell-casting encumbrance ended up with 60 fatigue.
- An Air-5 mage casting Wrathful Skies with 3 gems (he had 3 in possession, used all) with 8 spell-casting encumbrance ended up with 53 fatigue.
A shot of this Last one is included here. (His heroic ability is battle prowess (not reinvig).)

How can this be?

WC was the first spell cast, no summon storm power or anything. The mages were always accompanied by a SoS-carrier, but this one was always scripted to cast resist lightning first -- soothing songs etc. were cast later.

I was darn certain that Fatigue use (from manual) is 1/1+(CL-RL), so that a lvl-5 mage casting WC with two gems sould receive 66 fatigue plus the spell-casting encumbrance, so in this case: 84.
I was also certain that using extra gems gives you 1 more virtual level. So using 3 gems (instead of 2) should yield a fatigue of 50 + 8 = 58.

So is there a random element involved here?