Well 1080p is 1080 pixels wide, so obviously 768 is can't do 1:1 pixel mapping. there are 2 things you need to understand. Via HDMI and Component, TVs can only do 480i,480p,720p and 1080i. Newer models can do 1080p if they support it natively.
Via VGA, your tv is 1366x768. That means it has that native resolution. When displaying 1080i/p it downscales it, it shrinks the input to fit the screen. When displaying 720p it upscales it.
It all depends on the input device you want to use it for.