The reason why is fairly simple (although based on complex metrics): As great as Brees' performance was, it was one game in the large sample from which we draw matchup data. You should never make decisions, good or bad, based upon one game. Far too many things can go right or wrong.
For one, the variables counted on to define the game can change. The Giants' defense, normally a blitz machine, shifted to the same four-man rush, seven-man coverage scheme that they used to defeat the Patriots in Super Bowl XLII. The difference between that game and this one was that the Giants won many of the one-on-one battles on the line of scrimmage against the Pats, causing Tom Brady constant problems; in this game, the Saints' relatively unheralded offensive line held its own.
Without further ado, here are this week's projections: