Football is quite a bit more unpredictable than baseball and politics. Since it's a team sport, no matter how talented and well coached your team is one weak link or boneheaded play can completely change the game. For example, Flacco's pick 6 on the goal line by Harris isn't something that can be accounted for by statistics or past history, it was a bit of a fluke, and changed the game from probably 10-7 to a 17-0 game heading into halftime. That effected the entire game and changed both sides gameplans in the 2nd half (playing into ours, obviously).
I love me some Nate Silver, I just don't buy it.