A33Zab quoted: The altitude indications are based on GPS data. Two amber dashes cover the last two digits because the GPS altitude is less accurate than the barometric altitude.
I can (sort of) understand that back when the Denial Of Accuracy or dithering or whatever you want to call it was enabled by default on GPS. 100 meters was about as good as it got. Now it's about 10 times better between the DOA going away (I had a paw or two in that design) and basic improvements in clock and ephemeris accuracy. I wonder if TPTB have recognized this and given GPS a slightly better reputation and treatment.
Of course, I am presuming they have the intertial platform providing aiding data to the GPS receiver to help it track through extreme turbulence and other high dynamics conditions.