Today I tested against 2 Giants, 2 Trolls, 2 Windhelm Guards, 2 NPC's with armor but no helmet, and 2 Draugr Deathlords running 20 hits to head, body, arm and leg (arrow in the knee!
). The damage was always almost exactly the same for every body part and the variance, which was less than a point of damage, was often on the same body part. These were all done from the same saved position against the same target with the same character stats, apparel and bow.
I just posted to the UESP Wiki discussion page to pass on this information.