Here is a more everyday example: distance traveled by a vehicle and the amount of fuel consumed.

> Fuel efficiency can be expressed in terms of the volume of fuel to travel a given distance, such as in litres per 100 kilometres, or through its inverse, the distance traveled per unit volume of fuel consumed, as in kilometres per litre.

If you have a car that consumes 1 liter of fuel per 100 kilometers, it's the same as saying it travels 1 kilometer per 0.01 liter of fuel. They are equivalent, describing the same relationship and proportion. It makes no difference whether the quantity is a "deterministic" or "variable" measurement. You can use either unit, depending on the aim of your calculation.