Most display cards/monitors do not adjust for gamma -- which is typically about 2.5. TV sets do adjust for gamma but make an additional adjustment to correct apparent contrast in typical living rooms. Apple computers adjust for a gamma of 1.8 in order to closely match the response curve of the Laserwriter (The same laser engine as the HP-II and HP-III incidentally).
The lack of standard gamma handling causes a contrast deviations between different display subsystems and also causes color shifts for any color composed of mixes of unequal intensities of colors. Colors generated by dithering dots of pure color don't shift which may cause dithering to be used in some situations where it would otherwise appear to be an odd choice.
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