Honestly, after I went through and looked at the post-All-Star break run game-by-game, I think the biggest reason was that the pre-All-Star break record was pretty difficult and the post-All-Star break record was relatively easy.
The year-end record, 40 wins, seems fairly accurate when you look at it like that, even though the roster makeup changed a lot over the course of the year.
The Celtics also benefited from a good number of East teams that were missing key players (Bosh, George) or that were struggling despite playoff aspirations (Pistons, Hornets, Raptors).
If the East has fewer disappointing teams next year, it could really go the other way. But, on the other hand, the Celts might do a lot better overall against the opponents that creamed them in the first half of last season when the team was still carrying Rondo and Green.