[LDiCesare]

The mages very often use more gems than needed in order to reduce fatigue. If they had spent 1 gem, they'd have reached 100 fatigue and more or less been out for the rest of the fight. With 2 gems, the ai cuts down the fatigue to 50 and thus can keep them casting spells.
The best thing to do imo when you want to know what the ai "wastes" is to playtest it yourself beforehand. I realsied that an S9 pretender would happily pay 3 gems for a magic duel for instance...