There's a lot of luck involved in a football game. It's not even a round ball, after all; you never know where that oblong thing is going to bounce.
So while we want to judge teams based solely on wins and losses, quite a few underlying elements help tell us how teams will play in the second half of the year. Sometimes, teams get the benefit of pure luck: a defender dropping an easy interception, or an opposing kicker missing an easy field goal. There's nothing a kick returner can do when a kickoff sails over his head for a touchback, and nothing a defense did to deserve the gift of a blown snap that goes over the quarterback's head.