tl;dr: Humans perceive color logarithmically, and RGB channel values have only 8 bits of information, so computers take the square root of the brightness to eff

tl;dr: Humans perceive color logarithmically, and RGB channel values have only 8 bits of information, so computers take the square root of the brightness to efficiently store it, but most blurring algorithms take a naive average of RGB values resulting in blurring that is darker than it should be.