It is not an accurate formula, but it pushes you to the right way.
Usually the difference between the PMOS and NMOS threshold voltages is approx. 100 mV.
I doubt it is because of the mobility.
I guess it is because of the differences in doping.
I think, too...Possible to make a techology with same threshold voltages.
But I guess it is not so important and determine a technology is a trade-off between many things.
Doping level is very important in breakdown, resistance , etc.