IMO Djokovic was playing at his own limit at times last year. This year, he plays mostly within himself, and when he tries to push it to that limit, he has often made mistakes. To me that is normal in my experience (more so than the opposite, though it does sometimes happen). Not being able to bring ones very best in the biggest matches is also normal.
well, my impression rather was that top-players usually bring their very best in these matches.
And I don't have the impression that in the years before 2011, Djokovic was the kind of player who couldn't usually bring his best in biggest matches.
This year he tended to make errors when he tried to be more aggressive, it's true, whereas being less aggressive was enough to beat less good players, and of course it was more visible in big matches.