Football is often unpredictable. This would work better in maybe a seven game series but for one game.. anything can happen. Brady could get knocked Silly by J.J. Watt and all of a sudden he's not the same player and the Texans are exposing the Pats defense and there you have it... an upset.
OR a good field goal kicker can have a rough day.
OR the weather can play a big factor. What if it snows or it's slippery and nobody can hold their footing? all of a sudden a game that you would think would have more points becomes a defensive struggle.