为什么不调用IActionFilter.OnActionExecuted？

Question

public class FirstActionAttribute : FilterAttribute, IActionFilter
{
    /* Is Called */
    public void OnActionExecuted(ActionExecutedContext filterContext)
    {}

    public void OnActionExecuting(ActionExecutingContext filterContext)
    {}
}

public class SecondActionAttribute : FilterAttribute, IActionFilter
{
    /* Isn't Called */
    public void OnActionExecuted(ActionExecutedContext filterContext)
    {}

    public void OnActionExecuting(ActionExecutingContext filterContext)
    {
           filterContext.Result = new ViewResult { ViewName = "NotIndex" };
    }
}

    [FirstAction(Order =1)]
    [SecondAction(Order=2)]
    public ActionResult Index()
    {
        return View();
    }

OnActionExecuted()被调用用于在设置OnActionExecuting()属性的操作过滤器之前调用Result的操作过滤器，因此取消了操作方法的执行。在我们的例子中，FirstAction.OnActionExecuted被调用，而SecondAction.OnActionExecuted没有被调用。

a）如果 FirstAction.OnActionExecuted动作方法从未执行过，那么调用Index的目的是什么？

b）无论如何，由于FirstAction.OnActionExecuted被调用，为什么不执行SecondAction.OnActionExecuted呢？