You will notice that the display resolution may change to a lower resolution when you use the Presentation Display Mode keyboard shortcut (Windows logo key +P) to change the mode to Duplicate on a Windows 7 Computer with only one monitor connected. This issue typically occurs on different models and brands of video graphic adapters when connected via DVI or Display Port.
This is expected behavior with video adapters which have multiple DVI or Display Port connections. Even though you only have one monitor physically connected to the video adapter, there are still other DVI / Display Port connections which are not used but available. When you change the Presentation Display Mode to Duplicate, the system will try and synchronize with the video port connections and will default to a compatible video resolution of a non-DVI / Display Port connection.
To revert display resolution settings back to the previous setting. Use the Presentation Display Mode keyboard shortcut (Windows logo key +P) and change the mode to “Computer only”
Windows 7 Keyboard shortcuts http://windows.microsoft.com/en-US/windows7/Keyboard-shortcuts