How about maybe that the selection only really works to make people living further north develop lighter skin, so that they can make the Vitamin D they need in the pale winter sunlight there? Maybe there's no real need to darken up if you're light skinned and move back to the tropics.
Actually, there's not that much UV in the arctic. The amount of UV that gets absorbed by the atmosphere goes up very quickly with decreasing altitude, and the Sun never gets higher than 46 degrees even at the Arctic Circle - only 23 degrees at the poles.
I don't know if the details of this are online, but it was published in Sky and Telescope maybe ten years ago. The article might even be entombed in a box in my garage.