No idea what the elevation is all about.
Since this is measured by barometric pressure changes, it was probably the hull heating up/cooling (since the GPS was inside)
if you have a high end GPS unit it may have a true barometric pressure sensor...but chances are it's sensing it's position relative to the satellites to establish a 3D position (lat, long, elevation). Calculating elevation by GPS signal is less precise than latitude and longitude and the 11 foot difference is likely just the amount of error in the elevation calculation and/or a periodic weak signal.
