I understand some guns penetrate armour and thus negate it but then wouldnt that take my health down to 20?
In CS:GO, the killcam displays the protential damage dealt from the person who dealt the killing blow (AKA. the person shown on the screen), not the actual damage dealt, or any damage from anyone else who may have contributed to your death.
That's also the same reason why when you're killed from a headshot from an AWP, it says "4XX damage in 1 shot." instead of displaying "100 damage in 1 shot."
CS:GO displays the potential damage, not the damage dealt.
Protential damage = The damage that would be dealt if you had infinite health, except you only have up to 100.
Let's say you've came around a corner. In front of you is an enemy. But to your right (out of view) is another enemy. Let's say they both have AK-47s.
The first enemy (the one you saw) fires at you and deals 90 damage in 4 shots. At this point, you only have 10 health left.
The other enemy (out of your view) then manages to do a headshot to you, dealing 80 damage in 1 shot.
Although you have sustained more than one hits from multiple persons, the killcam displays the player who dealt the killing blow (which was the guy that was out of your view), and the potential damage, which was 80, even though it could only do up to 10 damage (as you only had 10 HP left).
Another scenario, but with only one person. It says "170 damage in 4 shots."
Potential ways this could happen is that the person was aiming at your leg (anywhere below the waist) until his 4th shot, which was to the head... Or maybe 3 pistol shots to the body and a hit from an AWP?
This is because someone else was shooting at you too. You only see the damage that your killer did in the screen still when you die, but not what others from his team did. If you enable the console, you get a full recap of your last death on the console output.