I good friend of mine just returned from a Precision Rifle class put on by Dan Flowers with LMS Defense. This issue was discussed there, and he relayed to me that in the long range community, this is a known issue. Something to do with the light you see from the target being reflected at an angle (think like a riochet) and not directly back to you. Because we don't actually "see" things, rather we see the light that they reflect, some pretty weird shit can start happening with different environmental conditions.