Urchin creates the Domains report by querying an approved DNS server that is configured to perform reverse look-ups. The DNS server does a reverse lookup on the IP addresses that are in the webserver's log file(s) and returns the corresponding domain names to Urchin.
The accuracy of this service depends on how fast the DNS server can respond, how much data is stored in the DNS server record and how many of the IP addresses are properly registered.
It is almost never possible to get 100% resolution of IP addresses to domain names. If you believe your numbers should be higher, you can configure Urchin so it makes more attempts to get this data from the DNS server. By default, Urchin will make 5 atttempts at getting 80% resolution. This value can be increased to 8 attempts and/or 100% resolution.
To change the setting, you'll need "Super Admin" access to the Urchin Administrative Control Panel. Then go to Configuration; Settings; Process Settings. Enter the IP address of the appropriate DNS server and select the DNS Lookup "On" button. After reviewing the results in subsequent reports, you can increase the DNS loops and Resolution Target as needed to increase the resolved vs. unresolved ratio.