I live at 3.7 meters (little over 12 feet) above sea-level (The Netherlands, the flat-pancake country) according to survey-details
So I can imagine with such huge differences the TPMS will have some substantial deviation on the "accuracy" of measurement.
(see also for example:
https://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html if it has your further interests)
Excellent, glad you like the GS911, I love it too!
I bought my GS911 back in 2011 and helped out many mates & sometimes even complete strangers.
With the amount of money I saved on "BMW bills" I can only imagine.
But then I'm a firm believer that when you help others and do good, you get good or help in this live and beyond.
I also believe that Jughead's findings should be added to the TPMS info-page for future reference.