Why do we consider all gasses to be in one phase?
Just because two fluids are completely miscible does not guarantee that they are uniformly mixed at all times.  

For example, if you have a glass half full with sugar water solution, you can gently pour pure water to form a layer on top.  This is the basis for sucrose density gradient centrifugal separation and layered drinks.

So the same can occur for gases, a non-equilibrium state can exist for considerable time where the gases aren't mixed even though they are completely miscible.