I wonder if someone, during design, might have thought that, for colors other than pure gray gradients (which don't occur much in real photos), using R, G, and B values where just one of the colors is slightly more or less than the others could emulate brightness levels in between actually displayable brightness levels. In other words, "it's not a bug, it's a feature"?
This, of course, would make more sense if things like the ProPhoto RGB rendering looked smoother than a pure grayscale gradient, but unfortunately it doesn't, really.
I've got to find a way to get my setup to present true 30 bit color. Seems like all the parts should be capable, but something's still missing. Maybe a special driver.
-Noel