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

[Peter_20]

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.

I used it against some pretty easy enemies close to Gatea.