I'm probably missing something obvious but... Should white skin not reflect sunlight more, and dark skin absorb it more? I agree that northerners have more need to absorb sunrays and hence make more vitamin D, so shouldn't they be black?
"White" and "dark" refer to the skin's effects on visible light. Visible light is different wavelengths from UV light; vitamin D production in the skin depends on UV light. "Dark" skin blocks UV light more than "white" skin does.
I think this is how it works: The melanin is not transparent, so it stops the light from penetrating beyond the very surface, and so a smaller volume of skin cells get access to sunlight.
To go a little deeper: Sunlight does two major things in our skin. It creates vitamin D, and destroys folate. Both are essential, so the various skin colors we have reflect the balance evolution has struck in various environments.
Incidentally you can test this yourself: if you go out in the summer you will tan i.e. your skin will darken. This is adaptation to lower the risk of sunburn and skin cancer.
If you could design a human from scratch, it might be most efficient to give northerners a D synthesizing pigment that's opaque in the necessary wavelength, resulting in the situation you describe.
Instead we have protective pigments near the surface, synthesizing structures underneath, and a need to balance D synthesis against UV damage.
Sure, some photons do get reflected, but the benefit is from those photons that get deeper into the skin.