From a chat with John Hollinger on ESPN today:
Will (NYC): I agree that some close games are 50/50 but those are in the minority. A big part of being a great team is the ability to show heart and win the close games. It's called performing under pressure and that is something that Boston showed they may be lacking greatly. That is why people are less confident in their chances.
SportsNation John Hollinger: (3:37 PM ET ) A lot of people believe that, but it isn't true is just overwhelming. Look at any team that was together for a number of years, even the great ones -- Jordan's Bulls, for instance -- and you'll find that the closer the score, the closer they are to .500. In other words, in games decided by two points or less they'd be almost exactly .500, even a team like the Bulls; in games decided by 15 points or more they'd be nearly 1.000. It's a fallacy that the good teams win the close games; the good teams win by 20. The lucky teams win the close games. There is no team in history that's been able to defy the correlation between scoring margin and wins over an extended period.
Statistical analysis has indicated the same relationship between luck and records in close games in baseball. It's a result that seems contrary to our intuition, which is that certain players, like the Jordans or Bryants, give some teams an edge in crunchtime. I believe in that idea generally, but still think that having Michael Jordan was that rare exception that did give the Bulls an edge in close games.