the line reflects these things(bear in mind even my grandma knows about GB / Favre's cold weather prowess):
AFC superiority over NFC:
Jack, even though from warm weather, has the style of play on both sides of ball conducive to persevering on bad weather conditions(their style is more an nfc/afc NORTH division combatant..
if the Under(due to bad weather) is likely, the lower the fave spread should be....