Two schools of thought working is all. A bullet that fragments will kill faster on a lung shot because it will inflict the most tissue damage into soft vital tissue and still have a good possibility of most of the pieces exiting.

A bonded or mono expanding bullet is designed to retain it's weight and mushroom. It kills very effectively too,destroying tissue both from contact and shock wave. The benefit however is that it will travel farther through non vital areas to reach vitals in the event of a hard angle shot.

Just to put it bluntly,I could shoot one in the ass with a Barnes and it exit his chest. If I shot one in the ass with a Berger,I'll likely just get a bloody mess. The deer might bleed out or might not.