That has to do with what i wrote before, i think.
Originally Posted by nick mulder
A histogram is nothing but a list of all possible tones, with information about how often each of those is found in an image.
So a gradient from black to white should produce a completely flat line histogram.
But only if the gradient tool is set not to favour one, or boths ends of the gradient. (An issue that can be be confounded by the image profile that a image processing software applies to that image - such a profile usually also contains a preferred gamma setting, i.e. applying the profile will change the tone distribution even if the gradient tool is completely 'true'.)
And if the gradient length matches the number of possible different tones (or a whole number multiple), so that it does not need to be made to fit. (A 256 tone scale from black to white, for instance, is hard to fit in a 200 pixel wide image without having to drop quite a few tones.)