ASP.NET MVC OutputCache vary by * and vary by user cookie

I have an asp.net mvc 3 project and I have a home controller. I have tagged my Index action with this attribute:

[OutputCache(Location = System.Web.UI.OutputCacheLocation.Any, Duration = 120, VaryByParam = "*", VaryByCustom = "user")]
public ActionResult Index()
{
    return View();
}

The vary by custom with user is handled in Global.asax.cs to check the user cookie value so that caching changes based on whether the user is logged in or not and what user they are.

When I go to this page on my web server, I get these headers in the response:

Cache-Control   public, max-age=120
Content-Type    text/html; charset=utf-8
Content-Encoding    gzip
Expires Sun, 20 Mar 2011 21:50:09 GMT
Last-Modified   Sun, 20 Mar 2011 21:48:09 GMT
Vary    Accept-Encoding
Date    Sun, 20 Mar 2011 21:48:09 GMT
Content-Length  3105

Right off the bat, the Vary - Accept-Encoding value looks wrong, shouldn't it have sent a Vary - * instead?

I am rendering the User.Identity.Name property to this view as well and I am noticing that even when I log out it will still render the user name, until the 120 seconds expires.

public override string GetVaryByCustomString(HttpContext context, string custom)
{
    if (custom.Equals("user", StringComparison.OrdinalIgnoreCase))
    {
        HttpCookie cookie = context.Request.Cookies["user"];
        if (cookie != null)
        {
            return cookie.Value;
        }
    }
    return base.GetVaryByCustomString(context, custom);
}

Been playing with this for several hours, and am completely stuck, hopefully someone has an idea...