Comment 9 for bug 467285

If gnome-screensaver isn't actually running, then the behaviour you describe is not a gnome-screensaver bug, and is also completely normal. Xorg has a built-in screensaver, which gnome-screensaver disables when it runs. If gnome-screensaver isn't running, you will see the built-in Xorg screensaver instead.

gnome-screensaver is running by default on a normal install. It's not running on yours because you've disabled it in Startup Applications