Unknown User in LogUser table

Hi, I am trying to im****t non-CS IIS log files (W3C Format) into CS DW. All

users are registered in Commerce server Profile and the field cs-username
is 
available in log files being im****ted (they are generated by SharePoint 
****tal server 2003), but something is not working properly and many
records 
in LogUser table are created with the field "UserName" set to "Unknwon". I
am 
also im****ting log files generated by a CS-based Intranet site and the 
intention is to create an OLAP Cube showing visits to both Intranet and 
SharePoint. 
Any idea ? 
Why are some records in LogUser table created with the field UserName set
to 
null ?? Why are requests to SPS created with the filed RegisteredUserID 
matching the null Registered User ? 
Thanks!