We have 12 registered users, using resources on the server. But the number of logged on clients is actually a lot higher.
I don't understand the difference between registered users and logged users

Do you mean some registered users are connecting from multiple workstation or multiple times from same device, or.... well, I don't understand.
Furthermore, I don't understand how this fits with LDAP authentication.
From my standpoint, either there is a process counting how many accounts are defined in Zentyal or there is some "session management" while accessing Zentyal based application but this aside, I don't understand how one could distinguish between registered and logged accounts.
I'm slightly confused...