Lessons are learned each week of the season.
Take the not-quite-completed Week 4, for instance. It taught us the Dallas Cowboys' defense is every bit as leaky as you think. That the San Francisco 49ers really need a healthy Jimmy Garoppolo under center. That the Detroit Lions should never feel safe with a double-digit lead. That the Buffalo Bills and Seattle Seahawks are going to make strong pushes in the championship race.
Because teams are always providing information, and because that information then adjusts football's hierarchy, it's almost always a good time to update the standings and power rankings.