This is based on my loose understanding of things and an assumption. The assumption is that this seems to be the case when the cluster is deployed to GKE.

Since I don't have Kafka manager installed, I will use the example of Kubernetes service, which has a similar port configuration in the console. This service is of type ClusterIP

Name         Cluster IP     Internal Endpoints
Kubernetes   10.11.240.1    kubernetes:443 TCP
                            kubernetes:0 TCP

The port 0 is added by GKE Ingress to randomly select a port for forwarding, as explained here and also related discussion here

In case of NodePort service, it is a different story.

Name         Cluster IP     Internal Endpoints
hello-web    10.11.249.126  helloweb:8080 TCP
                            helloweb:30193 TCP

This can be also seen in the service description. Since service is already exposed on a nodeport, there is no need to additionally expose on a random port.

$kubectl describe service helloweb-backend -n default
Name:           helloweb-backend
Type:           NodePort
IP:             10.11.249.126
Port:           <unset> 8080/TCP
NodePort:       <unset> 30193/TCP
Endpoints:      10.8.3.3:8080