You are right.
What's equally important is the full resolution.
Outside of blanking, NTSC media like DVD is 720 X 480i, whereas HDTV is 1920 X 1080i.
Generally, 960i is just 480i upconverted. So the actual resolution is still 720 X 480, just better defined and smoother being scaled to 960i. With 1080i having natively 6 times the picture information (after blanking allowances).