720p is a video standard that specifies a resolution of 1280x720 in progressive format (hence the name "720p") Apple, fortunately, seems to be sizing their screens based on HD formats, but giving some extra space for UI chrome.
Ah, good point. HD TVs are often (usually?) 1366x768. I guess they just picked that to provide room for chrome as you suggest? Seems an odd reason to go with 1366 and not 1360, and for video you'd think they'd promote using fullscreen anyway...