I do not understand why it is working

Hello, I am developing an ASP .NET 3.5 app with Visual Web Developer Express 2008 and SQL Server Express 2008.
 Well, I create a sql server database file (.mdf) and I write ADO .NET code for access to this database using Windows Authentication.
So, my surprise is that my application can access the database without creating a login/database user for NETWORK SERVICE account.
I understood that my application was running as NETWORK SERVICE, so I thought I had to create a login in the SQL Server and a user into the database (.mdf) with proper permissions.
 Can anybody help me? I understand why it is not necessary.
 Thank you.
 
NOTE: By the way, my website is IIS local based (I am not using Visual Web Developer web server).
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