Alternatively, it could be because WP7 is mostly focused on the U.S. and therefore has the weird two-year-contract-stagger effect going on. Couple that with the mass appeal of the iPhone and the ubiquity of Android on lower-end phones (remember, WP7 focuses on the high-end).
At that point probably close to 70% of all Android phones as measured by website traffic where clearly in the high end (Incredibles, Evo, Droid, Droid X, Droid 2, etc...).
If Android is dominating, it's doing from the top-down.
Or, perhaps they are attached to a platform: WP7 has some features for Xbox Live integration, which some groups (15-25 year old males I imagine) will find appealing.
If this is any indication it could imply that people with WP7 are not attached to the platform like iOS or Android users are.