Login screen wallaper doesn't fill screen

I have two monitors. My primary monitor has a native resolution of 1920x1200, my secondary monitor has a native resolution of 1680x1050.

I'm using the fglrx-updates proprietary driver, and I configured my dual monitor setup using the Catalyst Control Center.

When the login screen is displayed by Xubuntu 14.04, everything looks correct except that the wallpaper is displayed on my primary monitor at what looks like the same size as on my secondary monitor. It's stuck in the top left corner, so there are black borders on the right and bottom of the screen.

I would expect it to fill the screen, following the "zoom" behaviour that I have configured in the Desktop settings, which causes the wallpaper to show on both displays at different sizes, totally filling each display.