Maybe I'm confused here,
but I thought Blu-Ray used 24-bit color, i.e. 8 bits per channel (channels being Red, Green, and Blue), for a total of 16.7 million colors.
Which indeed means you're limited to 256 shades of red, green and blue (Mixed together you get 16,777,216 possible colors, 256 cubed).
So if you encode a videostream in color, instead of b&w, you can have more than 256 grayscale steps because you use colors in the mix.
If you downsample the first screenshot to 256 colors, and choose 'nearest color' instead of error diffusion or dithering, you'll see that some of the greys will turn slightly purple and green.
As far as the details lost in the shadows go, I'm looking at a properly calibrated monitor, but I'm starting to believe that it (the monitor) is set to dark or clipping the blacks. At any rate, I would still like to know which of the transfers use the correct brightness-levels.