Baro/GPS disagreement

I'm a little baffled at the huge difference between my GPS altitude
and the pressure altitude when I get up high. Yesterday on the
ground, I showed around 10 feet difference between the GPS altitude
and the pressure altitude. Up around 17000 feet, the GPS indicates
over 1000' higher. I also noted that the indicated pressure altitude
from the FR closely matches the altimeter (and the FR was recently
calibrated and no such huge errors noted). Is it possible that (at
least where I fly) the air just cools more rapidly than the accepted
model, lowering the indicated pressure altitude?

Jim