This is in spite of the fact that the Android phone sales are apparently equal the number of iPhones phone sales.
From what I understand, Android sales are currently similar to iPhone sales (in the US) in terms of phones being sold today, but there are a lot more iPhones already out there than Android phones (because the iPhone has been selling for longer I guess). Which means at the moment there are fewer people to buy Android apps. This also varies considerably from country to country -- for example, apparently Android sales are tiny in Australia compared to the iPhone.