The reason why you see many leather armored enemies getting killed by close range archers is due to the fact... They are archers. They are skilled in a bow much more than a sword. They are trained to seek out an enemies weakspots, aim for them, and anticipate an enemies movement, which is why they fire ahead of a moving target and not directly at them. With archers, the skill is before the initial strike, swordsman, the skill is after the initial strike. 

(Not including melee soldiers trained to deliver killing blows within the first strike). Example being duelists