You set up PBS sync to a remote machine, this same machine either already has a VM backup with the same ID, or will eventually backup a VM with the same ID. This will cause these backups to be grouped as if they are the same machine, this causes confusion and issues with purging if the rules are set on the PBS instance.
For me, I have a cloud server running PBS that already had backups under VMID 100 and 101. Once adding a sync job from a remote host, it successfully synced, but in the same grouping as the VM ID that was already there. This appears as extra VM backups.