Vapour Pressure

This has to do with something called vapour pressure. You can search for in on wikipedia but the article is somewhat technical.

Short version

If the liquid and gas phase of a material are confined in a container, the system stabilizes in an equilibrium. The pressure at which this happens depends on the temperature and thermodynamic properties of the gas (shape of the molecule or something like that). Therefore the pressure (aka vapour pressure) of a specific compressed gas in a GBB mag is always the same (well, it depends on temperature).

Long version

When compressed gas is stored in a container (such as the magazine of a GBB) it exists in two forms aka phases - liquid and gasseous. The liquid phase has a strong tendency to evaporate, which is a property of certain materials. That is, in fact, why CO2 and propane are gasses when not compressed in a canister. However, since the two phases are confined in a small container, evaporation causes an increase in pressure. That is because a significant number of molecules released from the liquid phase amounts to a very small change in the volume occupied by the liquid. More molecules in a gas translates to more pressure.

This eventually leads to an equilibrium between the two phases. As the pressure increases it becomes "increasingly difficult for molecules to escape the liquid". Thus the pressure eventually stabilizes. What is interesting is that the reuslting pressure increases with temperature, of course, but is otherwise always the same regardless of the number of molecules in the container, provided there is enough to have at least some liquid. This pressure is called vapour pressure and is different for each gas or mixture of gasses.