Finder allows rename on Exposing desktop
It's a bug. Basically it's not setting focus onto the desktop, the focus is still on the Folder/File that you're renaming in the Finder window, the only difference is the Window is being 'drawn' off screen. If you do it then press F11 to slide it back in the rename box points will be a perfect fit.

It probably shouldn't allow it, and I've never even seen it before, so kudos for finding a pretty bizarre bug!