Competitive Balance in the WNBA

With the start of the WNBA playoffs, I thought that I would turn my attention to competitive balance in the WNBA.  I am going to use a simple measure of competitive balance, the Noll-Scully metric which measures the leagues actual standard deviation of winning percentage as compared to a league in which wins and losses are randomly determined.  For those wishing to calculate this on their own, here is a step-by-step guide.

I am using the WNBA regular season standings data from ESPN covering the 2003 to 2015 seasons.  Below is a chart of the Noll-Scully during this time period.

As you will notice, the WNBA has experienced periods of high and low levels of competitiveness.  Overall the average Noll-Scully during this time period is 1.740, which is more competitive than the NBA and on par with men's ice hockey.

The question is why is competitive balance better in the WNBA than the NBA.