And wouldn't you have guessed it? It's all the 'instant level up's fault.
Zidane, Vivi, Garnet and Steiner are all with you at the start of the game, so
they're fine; they're at L1 and it's great. But when another party member
joins and you're at a higher level... well, they level up to match you. And
they *DON'T* get any bonus points for that.