From the A330 FCTM (my emphasis):
In cold weather, the atmosphere differs from the International Standard Atmosphere (ISA) conditions. The parameters computed by the ADIRS are barometric and ISA-referenced. The ADIRS use inertial data and barometric altitude to compute FPA. When the atmosphere differs from ISA, the altitude and FPA computed by the ADIRS and the associated indications on the PFD (altitude, VDEV) are not accurate.
When the temperature is lower than ISA:
‐ The true altitude of the aircraft is lower than the altitude that the ADIRS computes.
‐ The FPA the aircraft flies is less steep than the FPA the ADIRS computes.