To get an RGB value for an individual R, G, or B photosite location, you have to look at neighboring photosites. At the extreme edges, there obviously are no neighbors to one side. Nikon uses extra pixels at the edges so that they can run the same demosaic algorithm throughout the JPEG image (i.e., look at all neighbors). Some raw converters, however, will attempt to use photosites right out to the edges, using adaptive demosaic routines to try to calculate a reasonable RGB value when there aren't any neighbor values on one or two of the sides.