Ok, so here is just my thought, or experienced opinion. I believe the difference depends on the individual dog. Mostly on what they are fed. I have gone through many kinds of food, some leaving the hair and skin a tad darker. Season change's the coat also. I think they seem a bit lighter in the winter. Maybe because of the heavier coat.
I know with Jack, even spot size and location changed on his face for the first three years. Griz has gotten lighter, Dolly has gotten darker, Scrimps and Peggy Sue have gotten darker also, this summer. This is a very good question and something I will start paying closer attention to. But all in all I believe it depends on the individual dog, food and season's .