The altimeter is
calibrated to standard atmosphere conditions.
It is nothing more than a
barometer which measures atmospheric pressure and relates it to the altitude at
which that pressure is found in a standard atmosphere.
In a standard atmosphere
the pressure at 10,000 feet is 700hPa so if the static pressure (Ps) is 700hPa
the altimeter tells you that you are 10,000 feet and this is your pressure
altitude.