I see it the same way as procederual generation: it's a good starting point for an artist but will invariably need an artist to make specific adjustments.

Because averaging colors is arbitrary and the results depend on the color space. The tool they used averages colors in non-linear sRGB which gives unexpected results when doing any kind of color math like averaging and interpolating. They could convert to a linear or perceptually uniform color space like CIEXYZ or CIELab, average, and convert back to sRGB to get a more expected result, but it's still an arbitrary result.