It sounds to me like you are not mirroring the display but using the projector as a second separate display.
Check in System Preferences/Displays to see how it is configured. I would give you more details on what to check but I'm sans second display while I'm working on the road so my System Preferences doesn't show options for additional displays.
Check in System Preferences/Displays to see how it is configured. I would give you more details on what to check but I'm sans second display while I'm working on the road so my System Preferences doesn't show options for additional displays.