can,
I like your observation. I can't say what is happening exactly, but the fact that there is some change is reason to continue the experiment somewhat longer. Note that the steps and the noise that occurs on the pressure at the edge of switching across steps is caused by the Honeywell PX2 sensor. Apparently that is a characteristic of their signal processing chip about which others have complained for this series of sensor.
Anyone have a suggestion for a quieter absolute pressure sensor?