Sorry, but we don't know anything about the Health and Blood levels of the characters he's shooting at. And the guy at 8:00 wears a ballistic helmet, so that explains the two shots needed.

+ there is a hidden health variable (besides blood, bone and shock)

it doesn't regenerate and each time you get hit it goes down. Eventually it will reach 0 and you die. Doesn't matter if you are at full blood and no shock, it goes to 0 and you're done. Can be a single slap from a zombie that kills you.

The hidden health variable does not explain not dieing from headshots, ballistic helmets can, but a shot to the head without ballistic helmet is a certain death, the ghillie in the first video should have died because you cannot use head gear and the first guy with the beanie in breaks video should have died too.
And shots to the head with ballistic helmet you should go unconscious, it also depends on the helmets status, if it is ruined it is as if you have nothing so you should die, but unconscious does not happen with the second guy on breaks stream.