Wired
Have you actually burned a file onto a dvd disc and watched it on a dvd player or you are talking about video quality while previewing and actually rendering.
I have noticed that previewing quality is inferior than actual file burned onto a dvd, and dvd just comes out fantastic.
Most video editor while previewing or editing use a shadow copy which is in inferior quality than original work, this is done to make video editing fast and not to touch actual media file, and remember if your actual video was shot in non hd than you can not make it in hd through a software renderer.
Graphic card and cpu may also affect the final product