I think the difference is explained by the structure of my landing page. The iOS link is most prominent at the top, but the Android link requires some reading.

I'm going to update the Power 20 site to give equal airtime to iOS and Android.

Android sales have actually recently outpaced iOS sales because I'm able to iterate on the app's name more frequently in Android. I can find out which titles work faster on Android. With iOS, one must submit a new app every time they change the name. Crazy.