What brand and model television do you have?
If you're using VGA, you can try to force the resolution. Try lower the refresh rate, adjust the flush methods, and interlaced modes.
Many LCD panels often advertise 1366x768 resolution, but won't run it over VGA. This is a limitation of the panel.
If you have DVI or HDMI, consider switching to those.
1280x720 (720P) is what your screen is designed to run (as it is marketed as a multimedia screen). You may be stuck with that resolution. If you can find an option to enable 1:1 pixel mapping, you should be okay to go.
Though, from what you have said, it is possible 1:1 pixel mapping is already enabled and it's just putting in the black borders to compensate for it's native resolution of 768P.
Let me know how you go.