Excessive gem usage in combat?

[LintMan]

Wierd, in my current game, I supplied Luigi the Witch Hunter (2F 1S 2H) with 10 Fire gems, something I hadn't really experimented with before. And I'm really confused now about what happened...

I scripted him to cast Phoenix Power, Flaming Arrows, Incinerate, Incinerate, Incinerate. Those are all 0 gem spells except Flaming Arrows is 1 gem.

I sent him into battle, and because he gets my pretender's water 9 quickness blessing for sacred units, he gets quickness and the script is mostly thrown out the window.

Anyway, at the end of the relatively short battle, I check up on him and discover 9 fire gems were used, despite him only casting 3 rounds of spells!

I just went and reviewed the battle, and watched what he cast, and how many gems were used for each casting:
Round 1:
-- Phoenix Power - 0 gems, so far so good
Round 2: (now blessed, so he's casting 2x spells per round)
-- Flaming Arrows - 3 gems! Since Phoenix Power boosted him to 3F, shouldn't this just cost 1F?
-- Fire Fend - 3 Gems! This should also only cost 1F.
Round 3: Summon Fire Elemental - 3 gems. This is only a 2 gem spell

At this point, his fatigue was something like 162, and he's done for the battle, having cast 3 spells costing 9 gems total.

I suspect that those extra gems were somehow used to raise his level even higher to lower the fatigue cost, but I don't quite understand how that works. I had thought extra gems would be used to get a mage up to the level to be able to cast a spell, not beyond that.

Is there any way to control this excessive gem usage? I gave him 10 gems thinking it'd Last him 10 game turns/battles without needing to resupply from a lab, but he burned 90% of his gems in one battle! I can't afford to spend 9+ fire gems per fire mage per battle! And mages can only carry 30 gems - do I need to bring him back to a lab to resupply every 3 turns?

Yikes!