User.Identity.IsAuthenticated returns true but Session has expired !!

Here is an interesting one:
I have a page where, when the user has successfully logged in, I store some values in the session.
I then use these values to load an application list page. This page is automatically refreshed after 20.1 minutes, with the sessionstate timeout and forms timeout in web.config set to 20 minutes (with sliding expiration). This means, if the user does not use any page for more than 20 minutes, he is timed out.
My problem is that the User.Identity.IsAuthenticated sometimes returns true, when the session has expired.
Why is this happening, and how can I fix it?
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