As you could guess, there is a very complicated science relating to facial recognition. There is also a condition called congenital prosopagnosia where the person does not recognize faces easily. While my research was very technical, I did not find the answer I was looking for. I remember hearing many years ago that our brains have the ability to remember minute details of familiar faces. This is why commissioned portrait painting is quite unsettling for me. I fear they are looking for some subtly that I may not have captured in the painting.