I live in the northern suburbs of Johannesburg in South Africa. I have noticed while working with synoptic weather charts in geography class that the isobars indicate that Johannesburg should have atmospheric pressure of around 1010-1020 hPa, just like most cities around the coast, even though Johannesburg is at 1500 m above sea level. Weather service providers also indicate that Johannesburg's atmospheric pressure should be around that.
What confuses me is that my weather station at home (which I have been monitoring for several years now) never seems to show pressure ratings out of 840-860 hPa. My outdoor watch gives me the same pressures. I am convinced that my watch cannot be broken because during my holiday in coastal Cape Town, it gave pressure readings of the expected 1020~ hPa and I can "feel" the higher pressure that I am not used to.
Surely both of my apparatus cannot be wrong.
Is there an explanation for this?