FF2j - Why does my Ice spell increase by 4 points per use?

[Masamune·1600]

So, I use this Ice spell ONCE during a battle, and for some reason, it increases by four points.
I never used it more than once, and then I used my Fire spell once as well.
What's going on?

The formula which determines spell growth in FFII relies on the current level of the spell, the number of times said spell is used in the battle, and the "rank" of the opponent the spell is first used on. In all probability, you used your Ice spell against a high rank opponent, which would have led to faster growth.