It has nothing to do with damage type...
Headshot - 3xdamage
Headhit - 1.2xdamage
Yes it pretty much has
EVERYTHING to do with damage type. A pierce arrow (using old soak/resist values) which has several times the penetration of melee weapon means helmets are all but useless against them. An Armet Helm (60 armor) stops all of like 20 dmg from a headshot.
Horn Bow w/ bodkin = 26 dmg + PD5 (5*14% dmg increase per power draw) = 44.2 dmg
44.2 dmg - 20 dmg reduction(60 armor x .36 pierce penetration) = 24.2 x 3 (headshot dmg multi) = 72.6 dmg, enough to kill most people in 1 hit without them even being injured to begin with.
So even wearing best helm in the game, you can still easily be 1shot by some random gimp using non-heirloomed gear and average dmg bow.
Any devs feel free to correct my math.