In particular, I suspect the DACs and gamma buffers used to derive the LCD driving voltages may have some non-monotonic effects, which would definitely affect the test --- i.e. if increasing the numerical R, G, or B value actually decreases the corresponding colour intensity, even slightly, of the light emitted.
I did this a few years ago on a work LCD monitor. It was much more difficult than on a U2410 at home, on which it was mostly pretty easy. But even that was more challenging than on a GDM-FW900, on which the colors and the gradient were crystal clear, with no need to do any swapping.
