Create autopilot GKE cluster unexpectedly creates new subnet

Question

Good afternoon! Our team is trying to create a GKE autopilot cluster. For that we are using the below command:

gcloud container clusters create-auto my_cluster --enable-master-authorized-networks --network my_network --subnetwork my_subnet --cluster-secondary-range-name pods-range --services-secondary-range-name services-range --enable-private-nodes --region=northamerica-northeast1 --service-account=service_account  --master-ipv4-cidr=master_range

The cluster created does look good. It has all networking configuration details that we expect including its control plane range set to the one we listed for the 'master-ipv4-cidr' flag. However, we also see as a side-effect, there is a new subnet created inside our VPC. It has the gke cluster name as part of its name and its primary range is the master range. Why is this subnet being created? Is this expected behaviour? We searched docs and couldn't find anything about automatically creating a subnet for the control plane.