Interestingly there's a significant difference between Calibrated Altitude and GPS altitude in this guy (nor photo, just found it on FR24)
Calibrated 47,000, GPS 49,000
I wonder if this error is consistent and something to do with the unusual altitude or is it a local abberation? "Calibrated" altitude would derived from the air pressure, and is set using a standard ground reference of 29.92 for planes above FL180. The pressure at sea level there is actually around 30.1, but that does not seem too unusual.
To the topic, probably if the planes above were reporting GPS altitude then it would be higher that FL450, so if we are comparing two planes, we can't really compare Calibrated on one vs. GPS on the other.