while NTSC has 525 "lines" of resolution, quite a few of those are not shown. [link|http://www.udayton.edu/~cps/cps460/notes/displays/video_standards.html|Video Standards]
A standard American TV broadcast has 525 lines per frame, but only 480 lines or so are visible. About 10% or 45 lines are lost during vertical retrace.
HDTV standards chose to use what's actually shown when specifying the format.