Snakes don't always know they are dead. I killed a rattlesnake several years ago by shooting it once with a 9mm. I then cut off and buried its head and tossed the body into the back of the truck. I then spent about 45 minutes on a tractor while my daughter read in the truck. The snake squirmed the whole time and was still moving a little when I skinned it.
It could be that the first shot killed the snake but it kept moving and the shooter fired two more shots not knowing it was dead.