well, with my 29 str and 9 if head armor 60, im lucky if i survive HS...
Come onto NA duel sometime, let's see how many head shots you survive against my STF archer... maybe fraps yourself in battle and point out the times where you've taken a 100% damage headshot from an archer.
I'm sure it happens, but the archers that are able to score those 1-shot headshot kills aren't the ones that can effectively and consistently hit the head. I have no doubt that a longbow or rus bow user with 7-8 PD will be able to 1-shot most people with a headshot, but even if they are aiming right at the head there is probably a 30-40% chance they can hit the head with the shot at the range in my video... not even considering the fact that targets are moving ALL the time.
Basically what I'm saying is, a build designed to hit the head effectively still doesn't do a whole bunch of damage, while builds that CAN kill with 1 shot lack the wpf to do it consistently. Archers are between a rock and a hard place.