hopharrigan,
Don't know about the B747-400 but most aircraft performance manuals state the known error in the Standby Pressure Altimeter for various configurations and stage of flight. Part of the errors you observed would be position errors associated with the installation and the ability of the mechanism to accurately display the sensed pressure on the instrument. (hysteresis)
Let us be clear about this. When we fly at a given Altitude/ Flight level we are flying at a "pressure level" equivalent to that level if the atmosphere was standard. The only time our TRUE ALTITUDE and the indicated altitude are coincident is in the standard atmosphere.
The real significance of the DADC/CADC is in the ability to ACCURATELY SENSE and DISPLAY that PRESSURE. (called an Altitude or Flight Level). In cold temperatures close to the ground that pressure level is lower than that displayed by the DADC/CADC as a numerical figure.
RVSM requirements depend on a high accuracy of acquisition of the required pressure aka Flight Level and also digital Auto Flight technology of high sensitivity to capture and correct disturbances within very tight tolerances.
[This message has been edited by pterodactyl (edited 29 November 1999).]