If you send a billion electrons in then a billion electrons must come out or you build up a charge which can eventually lead to trouble.
My guess is the +12v wires are being load balanced, but the ground wires are simply connected to each other. As an example lets say each wire was rated to carry a unit of 1(going unit-less). Three +12v that are load balanced can carry a total of 3 units in. The ground units are simply connected together giving a variation of currents with the possible distribution of (.7,.8,1.5). One ground wire is now pulling 50% more current then its rated for. Add two more wires with the assumption that the two wires added pull the same current as the .7 above, then normalize the total current to 3: (.477,.477,.477,.545,1.02). The 1.5 current wire is now 1.02 which is just 2% over the rated current.