That is because of how the dual-screen setup you're using is accomplishing things. The GL drivers think the display surface is one the size of the combined screen resolutions, and the accelerator splits the rendering surface between the screens. When you ask for a fullscreen surface in OpenGL, it uses the WHOLE surface resolution that you're utilizing in dual-screen and then sensibly centers it.
You're never going to get "proper" fullscreen with a dual-screen setup. It is going to always be split that way. Either live with that gap in the rendering, get a wider screen monitor that gives you most of the dual screen setup, or do it in a window...
