I suspect QNH comes from taking the barometric reading on an altimeter that gives the correct height of the altimeter at the airfield concerned. Since altimeters do not read correct heights there will always be different QNHs between airports at different altitudes.
Now the question is why would it tend to be higher as you go to high altitude airports. Why does the pressure fall off less quickly than the altimeter expects it to? It must be that the airport at the lower level is cooler or the airport at the higher level is warmer than expected?