Saw this on AV Herald (
link)
[Crew was] reporting unreliable airspeed. The crew subsequenty (sic) inquired with ATC what their altitude reading were, the controller responded 200 feet. The crew reported their standby altimeter was indicating 6100 feet and according to GPS they were at 6200 feet, so they should be around that altitude.
Not a pilot. What might cause the discrepancy between the altitude that ATC was showing versus what the plane's standby altimeter and GPS were showing. Which instructions/reading should the crew follow? Don't know if they were in IMC or not.