Why May? Because this discussion appears to be based on recently released government transportation figures, which do lag by a few months. I haven't tried to find the DOT/BTS report, but I would feel confident that they are analyzing the data in a way that provides an "apples to apples" comparison. Instead, here's a quick link to a USA Today story on it:

http://www.usatoday.com/travel/news/...wa-ranking.htm