The head space in the keg will immediately equalize at 10 psi. However, what we care about is the volume of CO2 that will dissolve in the beer. This is why there are charts that tell you how much CO2 will dissolve at a give temperature and pressure.
Ideally you want the carbonating pressure and the serving pressure to be the same. This way the equilibrium from carbing doesn't change once you put it on tap. It doesn't matter as much at bars that serve the keg in less than a week, but eventually CO2 will come out of solution (or the keg will become overcarbonated) if the serving pressure is less/more than what the keg was carbonated at.