I've always said that death isn't an exact science. A .22 rimfire may kill a critter with one shot, instant death.. ...but shoot a matching critter of same size and location with a howitzer and it walks away.. well maybe not a howitzer, but you get what I'm trying to say.
My cousin shot his first buck with a .222, it pretty much died on the spot. That same season I got a smaller doe with my .270, hit it just about the same location-I ended up tracking mine for about 100 yards. ...go figure. My uncle has blown whole limbs off deer with his 300 Win Mag and they kept going.
Bullets aren't instant death rays, dispite what anti-gun people think.