2012 Prius C inverter+engine coolant drain-fill only one gallon?

I drained and refilled the inverter and it took roughly what the spec says but then I was able to also drain-fill the engine coolant with the rest of the gallon. In other words: engine coolant drain-fill only took about 3/4 of a gallon.

I drained the radiator and then the engine block and only got a few dribbles out of that, basically the tube I attached filled up but not much more.

I ran in maintenance mode until fan came on and it's still full so now I'm confused, why did it only take 3/4 of a gallon? I even lowered the car to horizontal to make sure everything gets a chance to drain.

Are the Toyota specs vastly exaggerated? Or do I not get that much when only draining by gravity?

Thanks,
Andrew
2012 Prius C 4
Oregon