"Pablo" writes:

Thanks for the input. I understand that true altitude is what the
GPS is reading. But why then would the elevation at airport
reference points be so highly accurate, while the reading at a few
thousand feet above the surface varies so much against the
altimeter??

It's inherent in the way barometric altimeters use an idealized model
of the atmosphere. Altimeter settings are specifically computed to
cause correct (approximately "true") readings at the surface. As the
actual departs from the idealized model atmosphere, so do the
barometric readings higher up.

- FChE