Internal bleeding drops blood pressure much slower than external bleeding.
Also, with a frangible bullet on a large animal, it's quire possible the chest is not "scrambled" and in fact the heart and far lung are intact or close to it. It all depends on the lottery of where the fragments happen to go, and that's why bullets that penetrate deep and track straight are better. That way you know if your shot is in line with the heart that you'll actually get the heart and both lungs. With a poorly designed bullet like the Berger, there is no such guarantee.
