Matchups. Different day, different level of performance.
There must be some unwritten rules of the NFL and one of them I would
nominate is that what Team A does to Team B is no indication of how
Team C will handle Team B.
OK, sometimes there may be general conclusions, but to me the matchups
mean everything, and that is what changes the outcomes to something
surprising. I'd add coaching and game planning to the matchups thing
too.
Certainly your game plan for the 45-3 game was very different from the
playoff game too, so even between two teams it's not possible to know
the outcomes in advance with enough certainty to bet your house.