No it's because they ran out of spell names. I know in english it is just Stone I - IV, but in other languages the spells have specific endings. In german it's -ra, -ga, -ka for example, I don't know the endings in other languages. These endings are used in basically all Final Fantasy games and Yoshi did not wanted to invent a new ending for the spell names.

It's also likely the reason why we jumped from Glare to Glare III / -ga, so we could end on level 100 with the -ka ending.