ASP .NET cookies / session

[SJ]

So, Here's whats happening -

In my web app, I set a cookie to expire after an hour(60mins) from the time
it was created. As far as I know, I have also set the session to last for
60mins.
FYI: I have a background (AJAX) request polling every 46secs.

The strange thing thats happening though is that 30mins from when the cookie
was created, the the expiration time on the cookie gets changed from the
specified end time to - "At end of session" and i get logged out. (I know
this is happnening because I have set Firefox to "ask everytime" a cookie is
set.)

I have checked the <forms/> tag in both web.config and machine.config. And I
have checked the session state timeout value in IIS. They're all set to
60mins.

So why is my session timing out at 30mins?

-Shefali.