There are already two conditions that we recognize as being disease states, that would facilitate this type of resistance.
Scleroderma - This is a chronic, rheumatoid autoimmune disease that causes thickening and hardening of the skin. (There are a number of other nasty effects as well, depending on the type, but this will just focus on the skin effects). If you have a population that is predisposed to this, or if your population develops just the skin portion, it would make it harder for arrows to penetrate, and would decrease the depth that they penetrate.
Factor V Leiden Thrombophilia - This is basically the opposite of hemophilia, in that instead of uncontrolled bleeding, the blood has an overactive tendency to clot. In "normal" biology, this presents a danger in the form of free floating blood clots (Which in turn can cause strokes and pulmonary emboli), however if it was task adapted to be faster/hyperactive clotting when exposed to air, it would also have the same effect.
So if you have a race that is predisposed to scleroderma and Factor V, then these could eventually mutate to be beneficial adaptations rather than debilitating disease states.