They were 10-6, not 9-7. Also, you're getting their defense last year confused with the one from 2 years ago.
In 2010, when they went 10-6, they were 9th in total defense, 5th in passing defense, and 2nd in points allowed. In 2011, they were 32nd (NFL-worst) in yards allowed, both total yards and passing yards, and 19th in points allowed. Their one defensive saving grace in 2011 was interceptions, where they led the league with a ridiculous 31.
And what do you know, the 2010 Packers, who weren't as good throughout the year but got hot at the very end of the season and had some balance between offense and defense went on to win the Super Bowl. Then in 2011, they went totally nuts on offense and ignored their defense, and they couldn't even win one playoff game.
Interceptions are a highly random stat and the Packers regressed to the mean starting with that playoff game against the Giants last season.