Marco Leon writes:

I thought the primary factor in determining the differences between
baro altitude and GPS altitude was the difference between "sea level"
and the WGS84 datum plane at the particular location?

In theory, yes. But in practice there is also a lot of inaccuracy in
altitude readings via GPS. The system is not designed to provide
highly accurate altitude, as the original users (soldiers in the
field, ships) didn't require it. For most users, it's a lot more
important to know where you are on the map than to know how high you
are above sea level.

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