User account that runs ASP.NET app

I have a strange case where Environment.UserName of an ASP.NET application (running at customer site) returns a name that I can't figure out where it is set.  Note that web.config file has no identity impersonate enabled (that is, this line in web.config is commented).  Typically in this situation (when there is no identity impersonate set), the application runs under NetworkService account.  When I looked at the application pool for this application it is using NetworkService.  But the Environment.UserName returns a different name.  Is there a setting somewhere in the ASP.NET application where this user is set (outside of web.config)?
Thank you in advance for any suggestions.
 
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