Ho MeGu Y wrote:

I've monitored the flights I've been on using hand-held GPS, and I can
tell you that flight altitudes rarely match up with what the pilots
claim to be their cruise altitude or in-flight map display. Being off
by 500 feet is common.

Altimeters based on air pressure are not as accurate as consumer GPS.

I think you're wrong. Consumer GPS units are not designed to move at
500+ knots. Also, GPS altitude is inherently less accurate than its XY
position. Also, there's the Flight Level factor already mentioned by
Pat...