On the A330 the static pressure ports are located such that the local pressure in flight differs slightly from the true static pressure. This pressure difference introduces an error in the measured altitude, which is corrected by the so called Position Error Correction (PEC). The PEC is a function of altitude and Mach and is presumably calculated in the ADIRU's but maybe not in the ISIS. AT FL350 and Mach 0.8 the PEC is about 300 ft.