"Matt Barrow" writes:

"Icebound" wrote:
[...]
If the altimeter setting came from a station in the valley 5000 feet in
true height below the aircraft, the indicated height could differ from true
height by as much as 1500 feet feet.

If it came from a station on the hill only 1000 feet in true height from
the aircraft, the difference is likely less than 300 feet. [...]

Mind telling me how that variation (of that magnitude) could come about?

If you run through the full "true altitude" calculation discussed
during early ground school, you'll see that there is a term that
relates to the elevation of the measurement station. The effect
is that the lower you are AGL, the closer the calibrated & true
altitudes tend to become, because deviations from the standard
atmosphere become less significant within less tall columns of air.

- FChE