well different guns do different types of damage, thats one of the main things.
In the coding a headshot does 150% of the base damage while body is basic 100%
Now perks do change that, stopping power reduces dmg taken by 25%, while the armor seems to be a bit bugged and counts full body + head as a extra 75% protection.
Me and sharpe use the armor penetration perk which gives us a extra dmg % of 50% on targets.
then again like i said it all depends on gun dmg, silencer reduces range and dmg taken by 25%.
Some perks do add up while others doesn't, and also as they said. Ping difference is a big deal aswell. Cos i've noticed myself that sometimes hits don't even register when they should have, because ping doesn't register the person there. Even if its only a split second different, it can mean between a kill or being killed.