The measurement should not be taken at the exit of the venturi! This diameter is very often larger than the actual section of the venturi especially in the case of an oval section.
Here is an example: on this carburetor with oval section if you take the measure at the end of the carburettor / venturi you will find more than 20mm: Is this a carburetor of 20mm? Of course NO !
2 other measure at venturi level gives us the dimensions of 15.33mm in width and 19.33mm in height. The surface is close to an ellipse but elongated, ie a surface of 235mm²: The actual dimension of passage of this carburetor is therefore much closer to 18mm (254mm²) than 20mm (314mm²).