As far as I am aware, the default settings on Android are to auto-update apps when there is an Internet connection available, and I thought that applied to all markets internationally.
I still have no idea why there would be any significant number of devices out there that have either changed from the default settings (and have turned off auto-updating apps), or have been offline for 3+ years until they suddenly tried your app.
Or maybe something else is happening. As I say, it's a mystery and I would be very interested if you could shed any light on this at all.