480, 720 and 1080 HD designations are defining the number of horizontal lines for that specific resolution. At 1366 x 786 you are getting 786 lines of horizontal resolution (from top to bottom). In TV terms you are just above 720p resolution.
Your resolution is called HD-ready. It must be 1920 by 1080 to be able to be called HD.
By the way, 1366 x 768 is different from 720p. 720p is in fact a progressive signal of resolution 1280 x 720, which is obviously smaller than your resolution. If your TV/monitor can process progressive signals, then your display can show 720p.
Some TV/monitors with 1366 x 768 resolution can even accept 1080p input signals from blueray players, but they downsize those signals to 1366 x 768 using their built-in scaling engines/chips.