OK, when looking at a circuit that is not complete, IE, no ground and therefore no current flow, you must keep in mind that your voltmeter is a very high impedance. This means that no work is being done by the load, (in this case lamp/lamps), when testing because the current is not really passing through the meter. A very very very small amount is, but is small enough to not really matter in this case. While in this state, to the meter, the intended ground side of the bulb/bulbs look just like the positive side of the battery so it will show 12 volts when referenced to ground. If the ground wire were intact and of a low resistance, the meter would show 0 volts because both sides of the meter are effectually connected to the negative side of the battery. If your meter is properly set up for and able to measure current in amps at a value high enough to operate the load, if tested without the completed ground side of the load and the negative lead of the meter is at the battery - terminal or some other grounded point, would show the current flow through the load and it, the load, would operate correctly. If the ground wire is intact you don't have a circuit with current flowing through the meter. Ammeters are a low impedance device and are fused to prevent damage to the meter, protecting them from an improperly configured test or a load that draws more current than the meter can flow. This is not considering the use of a clamp on ammeter that is intended to measure current flow in an operating circuit as they are not inserted into the circuit itself.
