Users are most likely to go out seeking new apps after a major upgrade or buying a new phone.
Users with older phones are much less likely to seek new apps
But even if you don't install new apps, phones still call the play store regularly at least to ask for updates of the OS and the installed apps so they should be counted in the stats.
Interesting. Makes sense, but are you aware of any data to back this up?
Only from my own apps.
New android versions are always much more likely to install my apps than their actual percentage of devices out there.