I have found this call only provides clients from "network" ie the Security Appliance. Where if you are looking at the Dashboard - Network-Wide - Clients default view without a filter, then you are seeing clients across all the devices in that network. You may see clients for example that have talked on a switch but not passed enough/any traffic through the appliance to show there.
On the Clients page you can filter to, Only security appliance clients, or switch, or access points. etc.