ISA 2004. Anonymous users in log

Hi,
Similar question i finded in this forum, but i not find answer for it.
ISA 2004. I created http access rule for some domain users, example "HTTP
ACCESS"
Then i looked in the ISA logs and finded some records with user name -
"Anonymous", rule - "HTTP ACCESS" and action "Allowed".
Why this is happens?
And how can i get rid from Anonymous records in logs?


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