We know how hard the airframes work to meet fuel economy and range guarantees they have made to their customers. Weight and power savings are paramount. It takes a lot of heat (fuel) to keep a pitot from freezing at 275Kt IAS and -40C.
Just maybe Airbus squeezed the specs just a little too tight, and ended up with pitot that are fine nearly all the time, but without enough reserve heating energy for every case of high altitude icing conditions.
Or maybe the pitot heat power supply or wiring is not up to the task, as suggested by this:
On 28 October 2009, an Airbus A330-202 (A330) aircraft, registered VH-EBA (EBA), was being operated as Jetstar flight 12... The airspeed disagreement was due to a temporary obstruction of the captain's and standby pitot probes, probably due to ice crystals. A similar event occurred on the same aircraft on 15 March 2009.
The rate of unreliable airspeed events involving the make of pitot probes fitted to EBA (Goodrich 0851HL) was substantially lower than for other probes previously approved for fitment to A330/A340 aircraft.