Where exactly are preps defined as being exclusive to attacks that can gain more raw power? There would be no point to preps if they relied solely on the inherent power of each attack, since that would mean that any attack that delivers more damage than an equally (or higher) prepped defense could absorb would automatically break through the defense. 

Going back to the armor piercing ammo example, this would mean that an AP round with zero preps could blow through a stone barrier with two preps behind it because stone isn't going to stop a depleted uranium bullet.