"I shot too high. The entire back of the skull of my Jew was torn off and the brain exposed. Parts of the skull flew into Sergeant Steinmets's face." source page 2 of the PDF and 78 of the book.

Shooting someone in the head isn't clean by any means. I believe what they were doing could be described as 'pith.' A term usually used to describe a humane way of dispatching animals.

A back of the neck shot severs the spinal cord and destroys the brain stem resulting in immediate death with little mess. It was the most efficient execution method and widely used by the NKVD. They weren't going for torture, it was efficient mass killing and head shots don't necessarily result in immediate or painless death.

The "neck" isn't what you think it is. As you can see in this picture, the pistol is aimed at the bottom of the skull on the back of the head.

This is a drawing of the brain and skull relationships. If you draw a line from the base of the skull forward thru the line of the eyes, it collides with the midbrain, which is where functions like breathing and heart beating are controlled. Without a midbrain, the body cannot live. If you tilt the head forward or the gun upwards towards the forehead, you still hit this important area.