Last fall, I was disappointed in my pressure-pro system because it reported tire pressures #4 higher the readings obtained with both an electronic and mechanical truck tire gauge. These readings were taken at 8300'. I am now at sea level and it is now spot-on with both the elec. and mech. gauges. I also have similar experiences with my Chevy trail blazer which showed a #3 differential ( to high) on the DIC compared to gauges. This doesn't make sense because altitude should effect all gauges the same??
Consider the possibility that the PP values might be affected by temperature. I am suggesting the possibility that the measurement system in the PP might be affected by temperature. You observed the variation under changing elevation conditions. Ambient temperatures probably changed as well. It will be interesting to know if you find any correlations. I like your practice of measuring with more than one device to verify that you are collecting "good" data.
We had a TPMS system that became completely unreliable at temperatures below 32F. It could have been battery issues in the senders or problems with solid state components that don't like the cold. I now rely on mechanical gauges, "eyeball" checks, a "thump stick," and an infrared thermometer, to check tires as we travel.
If you find a pattern, folk here would probably be interested in your observations. Many use PP to monitor coaches and towed vehicles.
JD: Thanks for your input. What mystifies me is that a similar thing occurs with my Chevy trailblazer Driver information center. It just may be that the mech spring loaded gauge and the Acupressure gauge do not compensate for altitude differentials. I talked with pressure-pro tech help and they couldn't come up with an answer either. I wont be back at altitude until the end of May but I will check the readings then to see how much the temp and height changes the readings