“According to this data, the plane’s altitude was only fluctuating by about 20-30 feet. This is more stable than I expected!”
Autopilots are very good and they are servoing to the pressure altitude.
Many pressure altitude encoders used in modern aircraft (for example to drive altitudes that transponders report to SSR radar or via ADS-B) have 25 ft encoding resolution. That 25ft resolution is likely what is being seen here. Other encoders have 10 ft resolution but 25 ft is very common.
Autopilots are very good and they are servoing to the pressure altitude.
Many pressure altitude encoders used in modern aircraft (for example to drive altitudes that transponders report to SSR radar or via ADS-B) have 25 ft encoding resolution. That 25ft resolution is likely what is being seen here. Other encoders have 10 ft resolution but 25 ft is very common.