Kinda freaky that the screen wasn’t accurate… good thing it wasn’t what the pilot was relying on.

There are two ways for a plane to read their altitude:

* Directly with some kind of fancy radar.

* Estimating, by measuring the outside air pressure and comparing it with a reference value. The higher up you go, the less air pressure there is.

This latter method is what is commonly used for flights: the plane is not exactly at 33000 feet cruising, but “is flying at the height where the outside pressure is value X.”.

The cockpit instruments switch from estimating to the direct measurements at a certain height (that is, when it is landing or taking off)

It sounds like your in-flight info screen didn’t make that switch.