MVC with Areas — Html.ActionLink returning wrong URL/Route

I am using MVC3 and have Areas in my application. In general, everything works fine, I can navigate to my area (e.g. Admin=Area, Controller=Department) like this:

<%: Html.ActionLink("Departments", "DepartmentIndex", "Department", new { area = "Admin"}, null )%>

However, what I noticed is that if I don't specify the area in my ActionLink, e.g.

<%: Html.ActionLink("Test", "Index", "Home")%>

This will stop working if I have navigated to the "Admin" area. i.e. my url is now
http://localhost/myproj/Admin/Home/Index
instead of
http://localhost/myproj/Home/Index

Any ideas on what is going on here?

My routes are all the defaults when creating an MVC app / Areas. i.e.

    public static void RegisterRoutes(RouteCollection routes)
    {
        routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

        routes.MapRoute(
            "Default", // Route name
            "{controller}/{action}/{id}", // URL with parameters
            new { controller = "Home", action = "Index", id = UrlParameter.Optional }, 
            new[] { "MyProj.Controllers" } 
        );
    }

And my area registration

    public override void RegisterArea(AreaRegistrationContext context)
    {
        context.MapRoute(
            "Admin_default",
            "Admin/{controller}/{action}/{id}",
            new { action = "Index", id = UrlParameter.Optional }
        );
    }

Is this by design?
This posting suggests using RouteLink instead of ActionLink:
say to use RouteLink
Is it required to have "area" routevalue on actionlink if the application was grouped into areas?