It gets clients names by sniffing the network traffic from them as it passes through. It watches things like DHCP requests and DNS name registration requests. Sometimes because of what client traffic passes through it this can take a while.
One interesting thing you can do is to track clients by who is logged into them (assuming you are using Active Directory). Try configuring:
Security Appliance/Active Directory
Once you have done this, on the "Network Wide/Clients" you can now add a column called "User" - which is the AD user logged into that machine.
