Weather is a huge missing component. Another is hilliness/other bike-friendliness (bike lanes, etc). The top 12 cities in the analysis are warm-weather cities (mostly California/Hawaii).
For example, no-one is buying bikes in Chicago at the moment, because it's freaking cold. But there are likely many more fixed-gear bikes here than almost anywhere else (flat, bike-friendly city with lots of hipsters). Portland is capital of hipsterdom - well atleast until Pittsburgh takes over (http://www.washingtonpost.com/blogs/arts-post/post/portlandi...) but it's cold there right now so people aren't selling their bikes.