As always RTFQ. Traffic asked the reason for the difference between GPS and the ADIRU altitude. The answer is that the two heights are unrelated - GPS being a measure of physical altitude while the altitude output from the ADIRU is a standardicsed estimate of altitude based upon atmospheric pressure.
GPS - either Traffic's own hand held or the aircraft's onboard GPS - indicate the height of the receiver above the WGS- 84 surface of the earth. What use this may be during cruise flight is debatable, for not all aircraft are fitted with GPS and a cruise altitude of more than 31,000 feet provides adequate safety height for even the tallest lumpy bits of the planet.
The ADIRU provides an electical output to drive an electrical altimeter or generate a pressure altitude reading on the PFD. During cruise flight this is standardised using QNH [29.92 inches of mercury/1013.2 millibars] as the atmospheric standard for ambient pressure at ground level. The actual pressure at ground level is most likely quite different, but the objective is to provide all aircraft with a common standard to work to.
Using a standard setting means that all aircraft indicating the same pressure altitude on their altimeters will be at the same level regardless of their physical altitude above the earth's surface - which is after all the objective of the whole exercise.