I noticed that as well, I assumed it might be linked to certain weapon types, but I never tested it. mostly because getting money in the game is kind of a PITA right now. Another thing I noticed is combat resources don't see to be getting used correctly. I believe the blind attack is supposed to cost MP but doesn't and another special attack I think I got at level 4 or 5 costs MP when I don't think it's supposed to, I think it was designed to use TP.