puzzle about unset req.http.cookie

[yangxu4000 at sina.com]

 Hello everyone,
I do not know where to ask my question  so I send it here. thanks everyone.
I hava questions about  req.http.cookie:I see some configuration on google like this：
if（req.url ~ ".(js|css|jpg|png|gif|swf|jpeg|ico)$）{unset req.http.cookie；}
it says if images have cookie，It can not be cached.
but I do not set this in my configuration, I still saw CACHE-HIT (resp.http.X-Cache = "CACHE-HIT") in http response.
anyone could explain  this?
thank you very much!

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