https://archive.is/D0FOH "Microsoft did admit to darkening the green hill" in the caption for image 1 (Bliss).
Based on the borders of the image shown being extended from the actual wallpaper file (take a close look at the top and left) it was probably cropped as well.
It's entirely possible the color was edited by mistake (i.e. converted poorly) - IIRC the color profile on the tiff was not sRGB.