Ff5 stats and skills pixel remaster

Okay so I was playing through and I had to change my classes because Faris had 50 in magic and I changed Butz to a monk. When I did that, Butz had a 2 magic stat. So I gave him black magic for thunder use but when I did that his magic stat went from 2 to like 30 or 40.

My question is this, why? Why does the stat change when I give him black magic and what other stats change with other abilities?