Why do I want 12-18 in of penetration and not maximum penetration along with maximum expansion?

I was looking at luckygunners tests of different calibers and ammo. It's known that the FBI looks for 12-18 in of penetration. I know most people shoot for those parameters. I look for maximum penetration over 18in as well as expansion. via /r/CCW http://ift.tt/2liFJWI