So let's expand on this theory: all battery metrics are calculated and faked. This second OS is so lightweight that it does not even affect battery life. It's a conspiracy theory, so it's assumed to be alien technology, the most secret one ever.

This second OS performs very few operations, such as recording and sending data to the network over time when it's connected to the internet. Or it doesn't even need an internet connection. It sends the information every six months, so you don't even feel it. The data is collected in a very lightweight, encrypted, zipped, alien technology way, so it's inconspicuous.