Market share /= number of devices.
Apple owns the "market", ie the money. Apple owners spend vastly more on their OS (via playstore). Android might be running on vastly more devices, but the people who matter, the rich people with money to spend on camera apps, spend that money via iPhones.
We live in a world where whales matter more than customers. The legions of android users who use their phones to make phonecalls, as internet browsers, and as navigational tools ... they don't matter as much as any single iPhone user who regularly drops a few hundred every month on eye candy iPhone backgrounds.