check out this link:
http://espn.go.com/nba/standings/_/type/expanded
Look at the team records for games decided by 3 pts or less and look at the records for games decided by 10 points or more. All teams have significantly more games decided by 10 pts or more than 3 pts or less.
There is no way the numbers you are coming up with are correct.
Maybe you are not understanding what I am really looking for or maybe I am losing my mind. I understand that 3.5 or so may be a typical point spread predicted by Vegas but that's not what the games are typically won or lost by....Vegas point spreads are the tipping point where they get 50/50. 50% favorites score both above and below that tipping point.
The lines are lower than the actual pt spreads that really occur in the games because the oddsmakers need to consider the underdog also has a chance to win.