While I cannot explain what/why this is happening, Quark has acknowledged that there is an issue with a box's fill when a tint is applied, not necessarily with how it is applied on an image as a colorant or background color, and the tint on an image agrees with the gradient fill on a box when using a tint. Quark needs to find the root cause and when that is identified there will be a fix in the future.
So for now I am happy that this doesn't affect PDF output, just on-screen. Trust the colorant numbers!
Take care, Mike