The current flowing in a circuit is determined by the supply voltage and the total resistance of the circuit.

Current (l) = Volts(V) ÷ Resistance (R).

By the same token power in Watts(W) = Volts(V)×Amps(I)

It is a convention to quote the power Dissipation of an electric element based on its notional standard Voltage.And designers and manufacturers are expected to ensure the electrical device will survive exposure to the normal range that any nominal supply can go through.

12V circuits are expected to operate between 10.5 to 14.5 V. Consequently the fridges 12V element should cope with that range of voltages without concern. When at working temperatures, 12V elements will have a resistance. So if the voltage rises, so will the current. In practical terms the change in current will be proportional to the change in voltage,and to all intents and purposes that will be good enough to estimate current draw, but scientifically, the resistance of an element will change in response to the temperature of the element, and an increase in voltage will cause more current to flow, but at the same time. more current means the element will disipate more power, which will raise its temperature which willcrease the elements resistance.

It is often assumed that heating elements have a constant Resistance, but in fact in common with most materials, the electrical resistance of a element does change with temperature. So the resistance of a cold element will be different to a hot one. How much it will change is dependant on the material used. In general conducting metals will have positive temperature coefficient and become more resistive with increasing temperature. There are some exotic materials that have a negative temperature cofficients