It's down to nominal analogue blanking.

The 8 pixels at either edge (strictly, PAL borders are 9 pixels but 8 is sometimes an easier number to work with) are to be treated as "garbage", and not included in any size/aspect ratio calculations.

HD doesn't have NAB, so an HD picture should be scaled down to 704x576 and padded (although 704x576 is also valid for DVD video, so you could skip padding it to 720).

Likewise, to display 720x576 anamorphic video on a square-pixel display, it should be cropped to 702 and scaled up to 1024x576 (a 16:9 frame). Almost equivalently, you can skip the cropping and scale to ~1050x576.

DE