It's because when you first pull out a gun, the hitbox for where the bullet goes is much larger than when the gun is out already. It's modeled realistically because when you first draw your weapon if you fired it instantly the aim would be not as good as when you had the weapon out ready to fire. In CS GO, the bullets do not always go where the aimmarker is, that is just a guide, in instances such as when you are moving, when you are firing quickly/jumping etc, even if the target reticle is on the target, it does not guarantee a hit. I agree that this is a bit of a bug, as in when you draw the weapon from its holster the target reticle should expand just like in other situations, but this is not unrealistic as you think it is.

I am LE (legendary eagle) which is tier 15/18 so I think I know the game fairly well by now.

True, but I'm replying in general to the other sarcastic comments about how inaccurate the game is. Most people who post stuff like that are probably used to being able to make jumpshots with sniper-rifles just because the targetting reticle is on the enemy. I was explaining that's not how CS GO works, and it's not like that for a reason.

he wasn't even aiming at him...