@cks deep-sleep logic at least somewhat makes sense to me, I think it's mostly turning of the peripherals and a special mode on the CPU similar to turning cores off.
While power back to last state is weird to me given how I conceptualize ATX power as "pushing the power button pulls ATX pin 16 (green) low and then mobo keeps low while on".
Kind of makes me wish someone like Ben Eater would explain it. ^^