PCIe Power Connector

My question is want to know depth information how different between 6/8pin.

As I know wire that used on the 6pin connector is 3 wire for +12Vdc and 3 for Ground and on 8pin is 3 for +12Vdc and 5 for Ground, So where is excess current come from? It seem like ratio between 12Vdc wire(rail) input and ground is not equal. Add another ground without add any of 12Vdc rail but got twice power?

Thanks you :)