I used libraw to decode NEF images and looked at the r, g, b values. It turns out that certain r and b values never occur. I.e., some histogram values are zero.
In fact, the pattern of zeros is very regular. So for red, we have no samples with values 5, 14, 23, 33, 42, 51, 61, 70, 80, 89, 98 ... etc. For blue, there are no samples with values 4, 12, 20, 28, 36, 44, 52, 60, 68, 76, 84, 93.
Every 9th or 10th red value is missing. Every 8th or 9th blue value is mission.
The images were taken in 14 bit mode on a Nikon D7000.
It looks like some kind of scaling artifact. Is this artifact something that is in the raw data, or is it a side-effect of how the raw file is decoded to an image array?