Actually, what Phil said is what I've always heard from just about anyone worth their salt in the world of guiding those who haven't done the job before. The hump, besides being a spot that is neither large in terms of lethal area, nor a certain lethal or even crippling shot, is also guaranteed to be challenging for a bullet. It is one of the most compact parts of the animal in terms of muscle, connective tissues and bone. It will seriously challenge - and often stop- many good bullets. Breaking the long bone or joints in both front legs will put the animal down right there. Missing will still be lethal in that spot (assuming the bullet is placed high enough so that a miss passes through the body).
