they base the spread on alot of things. First the home team gets 3 points right off the bat. So according to the vegas spread the saints are a half point better on a neutral site. Also their overall record, how they did against common opponents, injuries to players, what positions the injured players play, how severe the inuries are, and if the injured players are going to play.
Myself I love the fact the my boys are the underdogs, they always seems to play better the more of the underdog they are. I hope after they win this weekend that they are a bid underdog to whoever wins the AFC.