Defenses these days cannot nor should they be expected to hold every team under 20 points every single game. Not even the most talented & well coached defenses are able to do that anymore with today's offensive systems and the game rules heavily in favor of the offense. See LSU, Alabama, OSU.
At the end of the day the defense hasn't been completely lights out in every game but aside from a few bad plays they've kept us in position to win every single game. Time after time the offense puts the defense in bad situations because of their ineptitude. Today's offenses have to score at least 28 points every game (Not average 28 ppg) to have a reasonable chance to win.