Comment 2 for bug 1428076

In a Unity desktop, Nautilus is the component that displays the wallpaper.

Nautilus is a part of the Gnome suite, and the philosophy in Gnome is that you shall not run your desktop in a heterogenous multiple-display environment. There is one single integer-valued scaling factor to which Gnome applications respond; it would be no surprise to me if Nautilus internally does not save per-display scaling values for wallpapers.