nvidia xinerama login weirdness

When running with nvidia-legacy drivers, with xinerama and twinview enabled, I have a very weird small nagging problem. I want the desktop bar to run only across the left screen, so that the right screen has no bar across it and so all the icons are collected on the left screen.

When I log in initially after a reboot however, the bar spans the entire desktop, i.e. across both screens. Also, the login window (before I log in) is centered in the middle of the two screens, making it difficult to read.

However, if I log in and then immediately log out, the login window is now centered only on the left screen, which is how it should be. Then if i log in, the desktop bar spans only the left screen, as it should be.

Why does logging in and logging out again fix this?