Ask anyone why the Dallas Mavericks have the best record in the Western Conference, and the first guess will be defense.
Guess again. Under Avery Johnson, the Mavs indeed have become a much better defensive team compared to the Don Nelson era but that improvement actually happened a year ago. The Mavs finished 2004-05 ranked ninth in the league in Defensive Efficiency, my measure of a team's points allowed per 100 opponent possessions, and that standing hasn't budged in 2005-06. In fact, the Mavs are defending worse this year than they did in the 31 games Johnson was on the sideline (as either head coach or "acting" head coach) in 2004-05.
Yet the Mavs as a whole are playing much better basketball than they did a year ago. They were hardly chopped liver last season, winning 58 games, but this year they're on pace to win 66. That eight-game difference is perhaps a slight exaggeration because it partly reflects Dallas' performance in close games this year, but even in terms of Expected Wins, the Mavs are 4.3 games better this season.