The NTSC format is 704 x 480, 1280 x 720 and 1920 x 1080. This is what is broadcasted...your tv will then let you see it as-is or modify it a bit.
Your tv has a 4:3 aspect ratio(4/3)=1.333 so at 480p your looking at 640 x 480 (480*1.333), at 1080i your looking at 1440 x 1080.