I get terrific mpg, but I sure don't understand it.

Actually you've hit on my Spring/Autumn weather workaround (not for the faint of heart).
If it's too cool to allow EV mode on a "cold" start you can start to Ready mode, quickly shift into gear and press the gas for a couple of seconds then immediately pull the selector into N while you coast to your destination without the engine starting. This is the one situation in which I don't use the P button because the engine will start as soon as you push it. Turn the car off when you're where you want to be.

 

Okay! Lets try it this way.... If the EV light is NOT ON, the ICE IS

 

60 mph on highway, going downhill. Foot off the gas, then back on very lightly. EV light is off, but energy monitor says ICE is off

 

Problem is that the MG1 is forced to keep the engine spinning at that speed and your light gas application may actually be injecting a little fuel.
It's a fine line between on and off if the engine spins regardless.