Why are weather service atmospheric pressures systematically different from those I measure in Johannesburg?
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