720x486 is standard NTSC full res. When I load uncompressed video into an Avid - the video res is 720x486 - standard NTSC. It has been this way since the dawn of digital video editing in the early 90s.

720x480 is the standard res for DV or DVD. They lost the 6 pixels for compression reasons - but you'll only notice the difference if you load it into an editing system - you'll never see it on a TV.

Trivia: The N64 (and PS) only outputted 320x240 and it was upscalled to full NTSC.