The shockwaves have better ballistic coeficent. Bullets with betterballistics will hold better groups further out.
I'm gonna go out on a limb and say this isn't always true. I think the Powerbelt, being bore sized instead of .452 at nearly the same weight, has a smaller bering surface and SD so it may be getting over-stabilized and wobbles after a certain distance. This may be why 295-400gr Powerbelts and concialsare more popular than lighter weights in sabots. Just a thought..