Re:You could build a video projector (Score 1)

Whilst NTSC does have 525 "lines", it only defines 480 of those for active picture, the rest were left delays during monitor flyback (as the beam goes from the bottom to the top of ur TV), and other uses/future expansion (such as timecode, closed caption, teletext, test signals, etc).

So unless you want to view a lot of lines of blank/data/test signals you're only really interested in the 480 active picture lines.