Don't forget there are hundreds of app stores for android and you can sideload apps as well. In places like China very few download from Google play.
So actual download quantum is much much higher than ios.
As been said before, "downloads" means little in terms of boasting an app store's success. Me and a few friends could, in theory, spend hundreds of hours downloading every "free" app on Apple's App Store if we simply wanted to inflate Apple's download numbers. Since they are all free, it costs us nothing (except wasted time). Downloads is an idiotic measurement.
The sales revenue (let's say in millions of dollars) of a given app/online store is a much better metric, and one that is more relevant to a platform's financial monetization success.