True, that is a major part of the things that makes the illusion crack.
The example from yesterday was on a Invade and Annex server against AI. So yeah, that one time can suresly be of lag/desync (thats also a quite big problem to the game)
I personally don't often have this kind of problems with Arma 3, most of the times I've been playing I've gone for Automatic Rifles and CQB (or half long distance as it often gets to), and there I can live with 2-5 shots in the chest to put down. But since I "switched up" to Marksmen in the Swedish Forces I've started playing as sniper/marksmen in the game lately, and so far, aiming for "one shot, one kill" havent been a single success due to "immortal" AI (sure the go down 2-4 shots later, but it still sucks, the realism gets lost a little).
As Jona33 wrote, I can agree with, a major flaw is the visual part, that the just twitch a little and then can with only one round kill you a split second later.
I'm not gonna stop playing the game, I do like it alot, but this core feature of the game, does need a fix/remake.