Electrical conductors sometimes use copper for the hot leg and aluminum for the neutral and / or ground . This is true when they pull raw wire through conduit and when they put terminals on receptacles. The aluminum is cheaper, has a higher melting point, and the task is considered less critical to the purity of the voltage. Often amps have a 4ohm tap, an 8ohm tap, and a common tap. The common tap is ground. Same idea. Not saying I agree with it, just offering this as a matter of explanation.
