I think it's possible that the low-end Android tablets (e.g., the $99 generic ones, not the Nexus or high-end Samsungs) a) make up a huge majority of the number of units sold and b) are used in a very different way than the iPad is. For instance, I have plenty of friends who are Apple all the way except for cheap Android tablets that their kids use for Angry Birds or watching video -- and nothing else.

FULL DISCLOSURE: I work for Apple.