Two screens although there is no second connected

Hi,
I recently discovered that my 2008 Mac Pro (connected to a 30" Apple Display, Mountain Lion 10.8.4) shows two screens when I log in with screen sharing. I then tested it on the machine itself and yes, there was a second screen (800x600) in the upper right corner "virtually connected" to my MacPro - although there is just the 30" display physically connected.
This phenomena reappears even after a restart.
No, there is no second display connected via AirDisplay or so. When I look into the system profiler there is
It doesn't bother me but I still want to understand it. And maybe even disable it.
Any ideas?
This is a screenshot of the sharing session (the rectangles are inserted by me before uploading, they are not artifacts. Actually is it a screenshot of a movie).
Thanks in advance for any ideas!
Andy

Hi Malcolm,
yes it is.
Andy

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