There are other more subtle problems that may contribute. As an amateur photographer I've been told that most cameras are calibrated for light skin. If you aren't careful with your setup you'll get bad definition in shadows and relatively very dark areas. If you have badly calibrated/exposed photos then very dark faces, whether of sub-Saharan people or gorillas, may be poorly distinguished. I bet there are loads of unfair, hard-to-catch issues like that.
By the way I don't mean to say that you're wrong, but just to offer a Hanlon's Razor-type counterpoint as it occurred to me.