Simple: Because black people out West are exposed to other racial groups, (especially the white & latino populations that will make their kids lighter if they fuck), more frequently than they are in the Midwest, South, East.
There are a number of statistics urban geographers use to quantify this idea. The most common of these is known as the
Index of Dissimilarity. It is defined as "the percentage of two population groups that would be forced to move to give every geographic unit a share of each group that is equal to their share of the entire area."
The
University of Michigan Population Studies Center has an awesome database that is easy to use, you should check it out, there is excellent data and some very fascinating stuff. Generally, 0-40 is considered low, 40-60 moderate and 60-100 high.
Here's why skin-tone is definitely going to vary by region w/in the Afro-American Population:
Phoenix black-white = 48.5
Seattle black-white = 53.4
Portland black-white = 58.2
San Diego black-white = 58.1
San Francisco-Oakland-San Jose black-white = 65.4
Los Angeles black-white = 66.3
vs.
Chicago black-white = 81.4
Milwaukee black-white = 80.1
Detroit black-white = 84.7
Cleveland black-white = 77.8
New Orleans black-white = 72.1
Philadelphia black-white = 75.8
Does it matter, of course not. Just something that I have definitely noticed with my own eyes that is supported on paper.