To ensure accuracy, it is important to refer to base conditions when measuring the volume of a sample of liquid or gas.
The density of both gases and liquids depends on the pressure and temperature of the fluid.
This fact gives rise to the necessity of choosing a benchmark pressure and temperature in which all 'net' volumes will be expressed.
The number of molecules in a given gas volume depends on the pressure and temperature.
This is why the pressure and temperature must be stated in order for a volume measurement to mean anything.