FF2 - Why the hell did Firion's magic abilities increase?

[Peter_20]

Alright, this is boggling my mind greatly.
I was running around at Mysidia, gaining some levels and stuff in order to prepair for the Warship.
I have the somewhat odd party formation with Firion being the fighter, Maria the white wizard and Guy the black wizard (while Gordon acted like a fighter as well).
Firion was the typical fighter dude, with high HP and physical attacks.
He was heavily equipped, hell, he even had some stuff from Mysidia, like the Knight Armour and the Giant Glove.
He had all the potential of a tank, and zero potential of a wizard, and he attacked, attacked and attacked with his swords during this long battle.
When I finally won, I expected Firion to grow in HP, Vitality or anything related to a tank char.
For some reason, he developed in MAGIC!
His MP grew for 5 to 10, and his Magic Power increased by 1.
He hadn't used ANY kind of magic at all, except for the Ancient Sword (Curse), if that even counts.
STILL, he grew in MP and Magic Power.

What's going on?