Why are weather service atmospheric pressures systematically different from those I measure in Johannesburg? 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?
They apply a correction to the actual barometer readings and report what the estimated pressure would be at sea level.

Basically they use an imaginary column of air between the barometer and sea level and add its weight to the pressure measured at the barometer.

This lets them make pressure maps instead of just almost altitude maps.

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/pilot_handbook/media/PHAK%20-%20Chapter%2011.pdf

https://www.ec.gc.ca/manobs/default.asp?lang=En&amp;n=4349ABDA-1