.ASPXROLES Membership Roles Cookie Expiry?

Using ASP.NET 2.0, with forms authentication.
Just for a test, I configured the roles cookie in web.config like this :

<roleManager enabled="true" cacheRolesInCookie="true" cookieName=".ASPXROLES" cookieTimeout="2"></roleManager>

I wanted to see what would happen when the cached role cookie expired. Using Fiddler, after 2 minutes had elapsed, I could see that the raw value of the role cookie had changed.

I was expecting that on expiry, that ASP.NET would simply re-read the roles information from the database, and repopulate the cookie with the same value. So my question is, why would the raw value of the cookie change after expiry ? The cookie value is not human-readable (base 64 encoded and/or encrypted ?), so I can't tell if the information in it is the same, although the application still seems to work fine.

EDIT :

It looks like each time the roles are encrypted and cached in the cookie, it gets a different raw value.

e.g. if you run the following code :

RolePrincipal rp = (RolePrincipal) User;
string str = rp.ToEncryptedTicket();
Label1.Text = str;