The "Network Usage" report shows the traffic for every single IP/host which are attached to the UTM. A WWW request of a client from inside the LAN is shown as traffic on the LAN interface and also on the WAN interface where it goes through. If all clients in your network only or most time communicate to the WWW, then the WAN interface is of course the top user because it forwards the traffic of all IP/hosts in your LAN.
The "Web Usage" report based on the WebProxy, and therefore it shows only the traffic which goes through the WebProxy. Normaly the WAN traffic doesn't.
You can hide single IP address from different reports. To hide the WAN address from the "Network Usage" report, add the IP to
Report Settings -> Exceptions -> Reporting Exceptions: Network Accounting
Jas Man