If you're monitor is displaying 720 vertical lines then that's what you saw. Typically you see 3 sets of number for high-definition broadcast. 720p, 1080i, and 1080p. This is relation to the number of rows of information displayed on the screen and the method in which they are drawn. P means progressive in which each line is drawn in turn. I means interlaced which means the even number lines are drawn then the odd, rinse repeat.
My guess is that your monitor converted the signal to display it @ whatever resolution you have it set to.