Scare writes:

The strange differences between GPS-altitude and
pressure-altitude, here's Christian's in Google Earth with both.

http://ozreport.com/data/Christian_Ciech.20100723-155058.6034.12_GPS_and_pressure.kmz.

Green = GPS-altitude

Yellow = pressure-altitude with corresponding GPS-altitude

Red = pressure-altitude when there was no GPS-altitude fix

Blue = ground track

Looks like the pressure altitude is averaged over maybe 20 seconds and that has
the effect of delaying the readings by that much.

Red's show up in columns because soon after losing 3-D fix it also loses 2-D and
subsequent points are recorded as being at the point where the 2-D fix was lost,
while pressure altitude continues doing whatever it's doing (rising or falling).

See it also
here.