Why does my device shows a resolution other than that advertised?
You haven't specified the Android version, but on tablets the difference is generally because the system bar (or "combined bar") is 48px (specifically, 48dpi, but in your case that's 48 real pixels) and the apps are only considering available screen space when they report the resolution:


Portrait = 768 x (976 + 48) = 768 x 1024
Landscape = 1024 x (720 + 48) = 1024 x 768


Some references:


What is the true available resolution of a 10.1 Android tablet?
Asus TF101 tablet status bar steals 48 pixels
Metrics and Grids (from developer.android.com; explains why 48 dpi was chosen for UI components)