Ya we stray a bit ot from time to time.  On the pcie. Reason the 8 pin can deliver more is the extra ground.  If you have had a pcie burn it's usually the ground side.  Since one wire is used to sense voltage and the other two are ground.  That coupled with the fact most psu come with 3 hot wires in the 6 pin pcie. So it can technically feed more thru the 12+ then the ground can return.  Yes there is some burnt off as heat but most comes back down the return. And if that isn't the same as the 12+  then you have meltdown.  Seen this many times now and that's the best I came up with as to why the 8 pin can deliver so much more and doesnt add anything more other than two more grounds and another voltage sensor.

Best regards
d57heinz