The battery level your phone shows is just made-up bullshit. It’s roughly accurate of course, but they can’t really check how much charge is in the battery with 1:100 accuracy, so it just counts down at a roughly constant rate making adjustments to the rate based on rough measurements of broadly whether the battery is “real full” or “mostly full” or “almost empty” or whatever.

That’s not a conspiracy. That’s literally how it works.