Remember how Xion chipped Diesel, Malta, and Tia? Yeah. Xion's base, Gran Da Val, has in the walls sensors. These sensors amplify the HP and MP of any and every unit on the premesis.
Actually, they kinda work in the oppisite direction. Damage gets cut in half and spells cost less. Though this created a problem: Fractional MP costs and Fractional Damages. How do you satisfy this? He just made the illusion of more MP and HP within the base. When you go out of the base, the HP and MP merely round up, making it nice and neat.
Ever wondered why Xion's AIM address in the game is XIllusion? 
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