Apparently on the old avionics in the helicopter, the altimeter is calibrated by manual entry of barometric pressure to the nearest 0.1 inch Hg. So a rounding error could cause a 100 ft altitude discrepancy.
That's how it's done in all aircraft.
"The name pascal was adopted for the SI unit newton per square metre (N/m2) by the 14th General Conference on Weights and Measures in 1971"
It has to be manually set because there is no way to know what the reference pressure should be for altitude while you are in the air (the reference isn't necessarily the true sea level pressure since that changes constantly, it is what the SLP was at the time the reference was set). Using a non-pressure altimeter you can normally figure out what true SLP is, but the idea is that you want all planes using the same altimeter readings, even if it is not strictly correct.
All aircraft in a given area are told the reference by the controller when they enter the control area.