Why would a company put black lines around the picture in their DVD?
Old/small TVs tended to overscan their images.  The "title-safe" area was about 90% of the original image.  The DVD's format ensures that despite such overscanning, everything in the image is visible, including the titling shown in red at the bottom of the image, which is hard up against the limit of the frame and thus requires that the image be reduced so that a 10% overscan does not clip it off.