Any reasonable answer for huge battery discharge when device is off?

Nearly three years ago I have asked [this question](https://android.stackexchange.com/q/69477/20449) . It brought certain attention, but no answer. Of course there is no change in Nexus behavior and device discharges its battery up to 30-50 times faster when completely turned of than when it is turned on, but not used at all. To my complete surprise, after these nearly three years I have notice the very same behavior in many more mobile devices, irrespective of device type, manufacturer or even operating system. Recently I noticed this in: - Sony Xperia E, - Apple iPad Mini (first edition), - Microsoft Lumia 640 and 650. Is there any consistent theory or answer, why it is happening? Why device drains battery at all, when it is (should be) completely turned off? And why the discharge rate is so enormously1 high? I'm looking for an answer at least partially proven or grounded on some verifiable facts. Spy theories -- that it is due to fact that completely turned off device sends a lot of information to Google / Facebook / Apple / Microsoft / US Government -- are quite out of scope of this question. 1I'd say that even absurdly high, given the fact that device completely turned off can't stand more than 24 hours while turned on, but unused at all (lying on shelf) can last up to a week or at least a few days.