QNH is not regional pressure for sea-level. Important to remember this. It is the setting that, when the aircraft is on the ground, shows the airfield elevation on the altimeter.
This will differ from the setting if you were to dig a big hole down to sea level and drop the aircraft down it, mainly because of slight "errors" in temperature lapse rates and air density corrections.
Specifically, up a mountain it is cooler than it would be given the same terrain but lower down, hence the difference in QNH.