headshot kill depends on lots of things,like if youre crouching or standing up (better aim and diffrent target) on your weapon type (glock<AK47) if you have armor and helmet,if you're runing or not,sometimes terrain is important to but the most important thing is luck
and cs is diffrent for many many things,like your slower there=have more time to react,its weapons are less overpowered,maps are easyer to play because usually no one spawns behind your back and well,its a diffrent game