the saints are low [relative to their w/l record] because the type of ball they play vitiates the basic DVOA assumptions. the goal of the DVOA is to find the best team relative to the others [through certain assumptions and stats via league averages] because every team doesn't play every other team.
unfortunately [for DVOA], NFL football is not about which team is better. it's about wins and losses, and stringing together an unbeaten streak once you make the playoffs. DVOA has nothing to do with wins and losses.