If you have low armor value the enemy will have a much higher chance to 'roll' a 1 shot / decapitation hit because you do not have the necessary armor OR large enough health pool to negate their potential damage (per single hit).
Lets see a possible scenario (accurate to the best of my knowledge and understanding, so take it with a grain of salt)
You have:
You meet a Bandit leader, he is more powerful than your average run of the mil creature as after all, he is a leader.
Lets say the leader has the POTENTIAL to hit for 200 (with his nice 2 hand weapon)
You both come face to face, you get a couple of good blows off, but the leader is able to hit you with his massive 2 hand weapon dealing his potential 200 damage.
~Now lets break this down~
Outcome (OCM) = Current Health(HP) - Incoming Damage (DMG)
Incoming damage(DMG)= 200 (the value which the leader hit you for) - your mitigation (through armor)
as previously stated 250 armor = ~37% of physical damage mitigation
Therefore you only actually receive 63% of the leader potential 200 damage
And so;
DMG = 200 * 0.63
= 126
You ACTUALLY get hit for 126 damage
Assuming you started your ordeal with the bandit leader at full heath.
OCM = HP - DMG
=230 - 126
OCM = 104.
That is to say, after 1 hit you now have only 104 health points remaining, But you manage to stay alive.
But now, due to what ever reason you go hit once more by the leader and you get hit for the same amount of damage as you previously did and this results in you death
OCM = HP - DMG
= 104 - 126
OCM = -22
THEN there's a % chance that you will either be decapitated /special animated death or just knocked to the ground like a rag doll.
This is why you get 1 or two shotted and that is why you can 1 or two shot enemies.
Though the values are widely different in-game this is the simple mechanic that is in place, except that there are modifiers in place (ie on master difficulty enemies hit for 'xyz' amount more; eg 3.0 x more OR 300% more )
So what can we conclude?
Increased amount of armor = greater amount of damage mitigated (up to the cap of 85%, i.e your armor will only ever stop a maximum of 85% of the damage per hit)
Increased armor perks/ skill level = increased amounts of armor.
If you need to stop being 1 shotted you need to increase armor, increase the % of the damage your shield blocks AND OR increase your health pool (HP)
Now obviously, for some creatures their DMG will be too great for you to prevent them from simply 1 shotting you.
Sometimes the game is developed so that you werent meant to survive that hit and encourages you to use Different tactics in order to succeed (such as using a summoned creature to take the damage from the monster)