Yeah, to steal a quote from someone here, the league's scoring rate from the first few weeks of the season was unsustainable. Defenses were bound to close the gap a bit as the season progressed.
Here's my take on the old theory about "defenses are ahead of the offenses at the beginning of the season." I believe the theory was pretty much true but should've been amended to "the unit that the NFL rules favor is ahead of the unit that the NFL rules don't favor." For much of its history, NFL rules favored defenses. Somewhere in the past decade, that flipped. So, now given less prep time (early in the season, lockout, etc) offenses hold the major advantage.