If you look at many photos of natural metal finish aircraft you can see a subtle variation in tone between the individual sheets of aluminium that comprise the skin. Some of the sheets were of different alloys, thickness or just came from different batches. Recreating this tone variation on a skin is a good way to make the surface look more like metal and less like grey paint.
As I said this tone variation was missing in the Val photos I looked at. However Lyric1 has also found me some photos of B5N1 Kates with a similar silver scheme but on these there is a noticeable variation in tone between panels. So the early Kates featured a natural metal finish rather than silver paint.
