Calculating the annual energy consumption for a fridge
That 120W is the power while the fridge is actively working. Most of the time, the fridge isn't working - after all, it is a thermally isolated box and won't get warm by itself overly fast.

Hence, aside from marketing, the way they come up with that number is probably based on an assumption of how often the fridge needs to turn on its compressor to keep a given temperature on the inside, given a specific temperature (curve, even, maybe) on the outside.

The EU has mandatory power efficiency rating labels, the European Union energy label, which defines a  calculation standard for such things. You can read its text here. I didn't do that for you, because it's not clear to me whether the method therein applies to a fridge spec sheet for the Australian market at all.