A 1080 display doesn't mean that the video card is set to display at a resolution of 1080. You need to verify the resolution setting of the video card on the computer. On Windows 7 goto Control Panel, Appearance and Personalization, Screen Resolution.
Of course that is the first thing I checked.
Sorry, I missed in your post that you had already verified the resolution settings.
You haven't stated what the resolution is set to on the computers. 956 is the height report for 1280 x 1024 monitor on a windows pc.
Get(WindowsDeskTopHeight) on a Windows PC does not return the resolution for the entire screen, it returns the height of the MDI window.