The saying "on any given Sunday" exists because not all NFL rosters are created equal.
While hope springs eternal during the offseason, every team is in a different stage of roster construction. Certain organizations are simply far behind others that are contenders. While that can change over time, the idea that a few squads are consistently competitive is a far cry from reality.
So, yes, any team can surprise a superior opponent during one particular day of play. Just go back to lack season when the Indianapolis Colts, who finished 4-12-1 in a dysfunctional campaign, somehow handed the eventual Super Bowl champion Kansas City Chiefs a loss.