My guess would be they used a one-sided pcb to connect the cable to and used half the wires. Some sockets internally link the power and ground pins, so it works both ways, but you get no resistor network and thus only standard 5v which gives you 500ma max (at best). With the resistors connected by the cable it's about 900ma to 3a which is probably what happens plugged in "correctly". Or some other magic happens on one side of the PCB to fool the charger into pushing the full 3A.