It's in relation to a theoretical ellipsoid shape.
"The geoid can be as low as 106 meters (350 feet) below the ellipsoid or as high as 85 meters (280 feet) above."[1]
[1] https://www2.csr.utexas.edu/grace/gravity/gravity_definition...
"The geoid can be as low as 106 meters (350 feet) below the ellipsoid or as high as 85 meters (280 feet) above."[1]
[1] https://www2.csr.utexas.edu/grace/gravity/gravity_definition...
In this case it’s 106m away from a perfect circle. Almost nothing compared to 20km, but a real thing.