Let's not act like the Pats had a wonder defense. They were much better than Atlanta, but they were average. It's not like they were Seattle, who Atlanta hung 36 on.
Also it's of important measure that the Falcons have a lot of rookies and second year players starting. Jarrett 2nd year, Beasley 2nd year, Jones rookie, Campbell rookie, Collins second year, Allen second year, Neal Rookie
All of them improved through the season because they got more used to the NFL. Falcons spent most of the season as the worst defense in the NFL and moved up to 27th in efficiency by the end. That's a pretty drastic improvement. Which likely means the last few weeks of the season the Falcons defense was about on par with the Pats.