[Agrajag]

My guess: Fire Flies and Fire Darts both target "one person" and have 6/3+ effects. Fire Blast, for example has AOE 1.
When fighting against Niefel Giants, there should only be 1 giant per square, so while Fire Blast will deal 26+ AP damage to one giant, Fire Flies will deal 6 AP damage to 6 giants, and Fire Darts will deal 10 AP damage to 3+ giants. It is therefore possible that the AI considers this a better deal. Though in the case of Fire Blast it is more probably because it is range 5.
Fireball has a usable range, but is AOE1, 14+ AP damage, which again would target just one giant for 14+ AP damage, Fire Cloud would hit 5+ targets for 8 AP damage. The first could be considered weaker than both FF and FD, and the second is probably considered "not worth an extra gem over FF or FD".