Lurker,
If you hit too low for the spine, the critter may drop because of the shock to the nerve cluster in the neck. However, after they get their wits, they'll get up and run like h3ll. Far and fast. You've injured the windpipe and throat and the animal is doomed to die an ugly death. The hunters odds of recovering an animal hit this way are slim.
Shoot too high, again the critter will drop like a sack of bricks. But miss the spine high and the animal if allowed will regain their feet and be gone. This shot they are likely to survive. Again, the hunter will probably have trouble recovering this animal. Most likely will not in fact get this one.
Gotta hit the actual spine to kill quickly and anchor the animal without more shooting needed. Tricky thing is many people think the spine always is in the center of the neck. Not always so...