I sell an SEO app ( http://semtab.com ) on both iOS and Android, and the Android version is outselling the iOS one by a wide margin. It caught me a bit off guard because everyone I know keeps repeating that iOS users buy more than Android ones, but that just hasn't been my experience so far. I even put the iOS version on sale at $0.99 in hopes to increase purchases, but it is _still_ not resulting in as many purchases as the Android version at the original $1.99 price.
I think the root of the difference is indeed competition on iOS, and lack thereof on Android. The lack of any decent SEO app on the Android Market is the primary reason I made SEMTab in the first place. I think that is the biggest reason it is selling better there - it's easy to stand out when all the other apps on Android are total crap.
Or perhaps you discovered that there are more search engine marketers on Android than on iOS. Or that your app has more visibility on the Android market. Or multiple factors unrelated to the average user's general app-purchasing habits.