Being a game with very high level of attention ... - Personally I think it has more to do with the brokers trying to "cover" their behinds ...
Alot of people will probably be betting on the Packers to win (being at home) and Minnesota losing in Pittsburg ... and thus if the Vikings were to be favoured ... that could (not saying it will), but it could result in a large payback deficit ... - So by evening out the odds ... the brokers will be able to "forego" these bets some of the way ...
When odds are set you are using "statistics" more than ever, and by all logics, the Vikings should be "favoured" going into this game ...
Having said that, it's my speculation, and no matter what ... this game is one of the few I actually would wish I could attend in person ... alas it's a long travel (from Europe) ...