On a side note, I have never shot a deer with a rifle. Muzzleloader, shotgun, and handgun only.
I think sometimes people shoot a deer and it drops were it stood. They look at the entry and exit hole, dress the deer and it appears that the bullet was nowhere near the spinal column.
I think most likely in that situation, the bullet fragmented and one of those fragments damaged the spinal column.
This would especially be more prevalent in high velocity rifle calipers.
shotguns especially close create a lot of shock, but in a veterinary study of cape buffalo they found with a much larger number of animals than we normally get to shoot in a year that around half would go down from a shot to the heart and lungs area with the magnum round they were using and half didn't they were autopsying the cape buffalo those that went right down had brain hemorrhage those that ran didn't.
they came to the conclusion that even though all hit in the same place with the same big bore magnum cartridge what mattered was if the heart was building pressure or releasing it
think about your blood pressure say you are 120/80 that if the pressure is at or near peak then the pressure wave causes the hemorrhaging.
this is a function of volume , if you shoot a sub 50 pound animal you can get shock every time you hit the chest and as you get larger and larger animal you get less and less consistent in the shock.
run a higher power rifle dumping more energy and you can again get shock from the pressure wave.
if you never shoot deer over 90 pounds and use a 270 you would likely never have anything but shocked deer on a decent hear lung area shot
but when you shoot a 225 pound deer with the same 270 you will likely have more mixed results especially if you add range
slow that down and and shoot them with a 30-30 and your less likely yet to get shock
20ga foster slug at 50 yards or more is more unlikely to get any type of shock as it is moving sub sonic already.