Question about Cookie slidingExpiration="true"

I use Forms Authentication with Cookies and place slidingExpiration="true" in my web.config and watch the http stream. I noticed that the AuthCookie is sent to server by the client in every request. But the server didn't send the lifetime-extended cookie for every single response except for certain response.
I am wondering when the Server decides to extend the cookie lifetime and resend it to client? And how does the Server implement this?
thanks.
...