Trusts' AI, Exposing unknown Weakness and Resistances?

[Zeargi]

So, I haven't used him on every mob, but Robel-Akbel is becoming one of my favorite offensive casters. He'll stun, use decent WS, analyze an enemies weakness, and Combo -ja's spells VS strong enemies.

but I recently used him VS enemies that have elemental weakness I didn't know or simply of spells he doesn't possesses. In most cases he goes for the Coup de grace with his highest tier spell that will be unresisted. In the case of the Flans he used Thunder V, for Matamatas He'd cast Aero. But out of the blue, I decided to farm somethings for cooking and I fought some Brasscaps. This time He'd cast Stone V, and continue to cast downward sequentially until he reached Stone II, where he quicking shifted to Fire V, and then back to Stone spells again. I know that Fungaurs are weak vs Light, and resistant to Dark and Water; Using Adelheid confirms the Light weakness as she's the only 'BLM' type that casts Light based Black Magic. This still doesn't explain why Robel-Akbel would choose Stone V over Thunder V, though? Anyone else come across strange findings?