The title pretty much says it:
If the shot is lethal the feedback is pretty instant. However if the guy you shot only gets some blood and shock damage he can run for quite a while until he drops unconscious. I think the server has problems making the proper calculations with blood loss and shock damage. Therefore: Desync happens. The player on the damage receiving end has even time to shoot back even if his shock levels would indicate that he should be unconscious.
Headshots (if lethal) drop players instantly. There is no desync to be experienced.