This is about visual perception of luminance, which to the eye is comparative, ie takes account of the lighter or darker background around a specific patch of the picture. The computer on the other hand measures the absolute values, which I have recorded above.

You can get do an equivalent simple exercise on human perceptions of heat compared to thermometer readings by filling 2 bowls of water with cool and fairly hot water. Dipping your hands in in different sequences gives you a very different impression of the temperatures of the 2 bowls than if you stuck a thermometer in.

BTW no spelling mistakes as I am taking notice of FF2 underlines. Try the counter-intuitive approach that solved my FF problem ie if you want the spell-checker ON, TURN IT OFF, quit and restart. Now turn it on & next time it may work.