I had noticed this about dogs many times in the past, and finally got the chance to snap a good example. One of the first things we stress to beginning horse artists is that the white patterns overlay the body color. The shading does not follow the outline of the spots. Pixel artists even have a name for this kind of artistic error, where the outer edges of a shape are dark and the center is light. It is called “pillow shading“.
Yet for some reason, sable shading in dogs often does exactly that; it concentrates along the edges of the colored patches. Notice the dark area along the topline of the Whippet above. Sable often does that, but look at how it travels down the edge of the two hindquarter spots. Just so it’s clear that the sable shading didn’t just happen to darken at the hip there, here is the other side of the same dog.
Here the white occurs a good distance forward, yet the same dark edging is visible on the border. Using Photoshop to remove the white patch and recreate the dark area underneath, it’s even more obvious that the dark area would be oddly placed were it not related to the white patterning.
I have no idea why black shading behaves like this in dogs but not horses, but it does go to show that artists have to look for these kinds of details if they want to capture animal coloring accurately.