Little scenario that help to understand what i'm saying:
Kefka: You cannot kill me, because i controll all magic in the world!
*Someone attack Kefka with a magic spell not related with the Godesses*
Kefka: Crap, i don't have controll over your magic!
Sephiroth: You cannot kill me, because i controll all magic in the world.
*Someone use a Materia to manipulate the pure Lifestream, creating magic, and attacking Sephiroth with it*
Sephiroth: Crap, i don't have controll over your magic... yet.
*Someone use a Materia to manipulate the Lifestream, creating magic, and attacking Sephiroth with it... But nothing happens*
Guy: Where is my magic?! Why nothing happened?!
Sephiroth: Because the part of the Lifestream that you was using to create magic, is already corrupted by me. Your magic now, is my magic.
Guy: Oh crap!
*Guy die*