[Etaoin Shrdlu]

Fire Flies has 6 shots. Fire Darts is 3+ (so an F1 would send 3). The AI assumes (I guess) that "hey, I have twice as many chances to hit!" without noticing that the Darts are bigger hits and better targeting.
Try with an F2+ (gets more Darts at higher levels.) I imagine an F4 would not use Flies at all.