Then changing from RGB to CMYK and back again would change that, wouldn't it? But will it? It's an image file, viewed on, and made for a computer screen. So it's always additive.

Histograms do show distribution of brightness. Not necessarily of the original, but certainly of the file, i.e. of a converted representation of an original.
You are saying, Mustafa, that for each of the colour channels contained in the file's colour mode, the histogram is flat?