It is not a "web service function". As far as the client is concerned, it's a set of descriptions in a WSDL document. From that description, client code is generated. That client code causes XML (SOAP) to be sent to the service.
It is possible for this to get out of sync. For instance, version 1 of the service may have had 3 parameters on the operation. A client was created for version 1, so it sends 3 parameters.
Version 2 of the service may remove one parameter. If the client is not updated, then it still will send 3 parameters. The service may choose to ignore the extra parameter.