now please correct me! but if you are telling the altimeter that the pressure is 'less' wouldn't you expect the thing to show a 'higher' altitude as pressure decreases with height??
But you are
not telling the Altimeter that the pressure surrounding it is less. You are telling it that the pressure at the Base Line (eg Sea Level) against which it measures the difference in pressure is less.
If the pressure, at Sea Level, is 1012Hpa and the Altimeter
is at 2,100' then the pressure surrounding the Altimeter will be approximately 942Hpa.(70Hpa difference as 1Hpa equates approximately to 30' difference in Altitude)
Then, if the Altimeter Pressure Sub-Scale is set correctly to 1012, the Altimeter will indicate 2,100' because it is measuring the pressure difference (70Hpa) between where it actually is and the Base Line YOU have input.
If the Altimeter is incorrectly set to 1011Hpa then it will indicate 2,070' because the difference in its surrounding pressure and the Base Line YOU have set is now only 69Hpa.