Armour is meant to suck at protecting you. You can eat some low end weapon hits or hits that don't have much momentum to them but Anything moving fast enough or piercing enough is going to ruin your day.
Blunt weapons by and large ignore armour and anything with a high piercing value like Crossbows still hurt you alot.
Your movement prior to swinging affects damage. You get a movement bonus.
So help you god if you get hit by a couched Lance, thats instant death 4 times over in most cases.
I saw a noble survive a couched lance to the spine once. I was intimidated.
But the real problem is, armor doesn't stop the big cutting weapons very well, either. A good sword swing'll kill a level 20 dude in plate if it's two-handed.