直接访问操作时找不到命名路由，但通过AJAX访问时找到

我有一个命名的路由：

routes.MapRoute(
    name: "OfficeByZipCode",
    url: "RetrieveOffice/ZipCode/{zipcode}",
    defaults: new { controller = "RLO", action = "RetrieveOfficeByZipCode" }
    );

我正在检索操作中的命名路线：

public ActionResult RetrieveByZipCode(string zipCode)
{
    try
    {
        Office obj = null;
        string urlOffice;
        //build a route dictionary that includes the zip code
        RouteValueDictionary route = new RouteValueDictionary
        {
            {"zipcode", zipCode}
        };
        //build the relative path for the route to retrieve JSON data
        VirtualPathData vpd = RouteTable.Routes.GetVirtualPath(null, "OfficeByZipCode", route);
        //combine the relative path with the site's root path
        //use the config value rather than HttpContext.Current.Request.Url to overcome issues that arise from Load Balancers and SSL offloading 
        urlOffice = String.Concat(Properties.Settings.Default.RootUrl, vpd.VirtualPath);
        //the rest of the code. not important for this example

现在，我通过两种方式之一访问此操作：

一个，通过AJAX，来自不同的页面：

$.ajax({
    url: "Test/RetrieveByZipCode",
    type: "POST",
    data: JSON.stringify(formData),
    dataType: "html",
    contentType: "application/json; charset=utf-8",
    success: function (data) {
        $("#content").html(data);
    },
    error: function (error) {
        alert("Error");
    }
});

二、直接通过浏览器：

http://localhost/RLOService/test/RetrieveByZipCode/92677

当直接访问操作时，此行返回空：

VirtualPathData vpd = RouteTable.Routes.GetVirtualPath(null, "OfficeByZipCode", route);

但是，当通过AJAX调用相同的操作时，它工作得很好。

两者之间的明显区别是，一个是我通过POST（AJAX）访问，另一个是通过GET（URL）访问。如果我将AJAX操作更改为GET，我会收到同样的错误。为什么这会有所不同？