It's pretty simple:
If I turn off the phone's screen and then connect the wall charger, the power supply draws ~6W from the wall, and this doesn't change if I keep waiting.
If I connect the wall charger and then turn off the phone's screen, the power supply draws ~10W, and I hear a very slightly audible high-pitched noise from the power supply the moment I turn off the screen.
If I turn on the screen again, it goes back down to ~6W, but once it turns off it goes up again.
I have two questions:
What is causing this?
If it is intentional and not a bug, then what is the rationale for the cause? Is this well-known?
The ROM is CM11S 05Q.
(Yes, the charger & cable are the OEM's, and yes, I've actually measured the current.)