Both Apple and Google count only unique downloads of apps per user account. This means that if an app is universal, it is only counted once, no matter how many different devices you may install it on, as long as all of the devices are using the same account. Neither company counts pre-installed apps, so Google Maps, Talk, etc don't count, but iBooks does count because that is an optional install.It turns out that both companies count app installs the exact same way.
When we first reported Apple's announcement yesterday, we noted that the iTunes App Store had a 4 month headstart on the then Android Market (now Google Play), not to mention the lag in time between when Android actually had enough user support to really be a factor in app installs. According to the counting method, we should also add in a caveat because iOS tends to have fewer universal apps, and more apps that are specific for iPhone or iPad, which would count as two downloads if the same user downloaded both. Although, that may not add up to much.
Either way, it's clear that Google Play is growing faster than the iTunes App Store, and based on Google's own numbers, it should pass the 50 billion app mark in less than one month. But Play still has a bit longer to go before catching up to the App Store in terms of revenue.
source: The Next Web