Variation between the Meteo Data of NASA, METEONORM with the Actual Values measure at site by Pyranometer

[bolla suresh]

I have read that the values using Satellite data given by NASA, Meteonorm used in the PVsyst simulation are generally 10-20% higher than the actual values measured using Pyranometers at site. Is it true?

If it's true, then are there any measures taken in the PVsyst Simulation to minimize this error? if yes, then what are those measures taken by PVsyst ?