I imagined complex things and missed the obvious.
If the DMG reflect is simply mirroring part of the damage you take and buu is a tank (so you want to reduce the damage he takes as much as possible), the more tanky the buu gets, the weaker his passive becomes?
Edit: Damage reflect working this way can't compete with HEALTH%, DMG Reduc% and HP Regen% for a slot on equipment, it's almost like the character has no passive since you want to avoid being at 50 or 70% health. Also, why does this bonus exist? I have never seen equipment geared towards this in city houses.