为什么ajax请求作为非ajax发送？

Question

我有以下索引视图

@model VirtualCampus2.Models.EmployeeViewModel

@{
    ViewBag.Title = "Index";
    Layout = "~/Views/Shared/_Layout.cshtml";
}

<h2>Index</h2>

@using(Ajax.BeginForm("GetEmployee", "Employee", 
    new AjaxOptions 
    {   
        HttpMethod = "get", 
        InsertionMode = InsertionMode.Replace, 
        OnSuccess = "updateDivList", 
        UpdateTargetId = "divList" })) 
{     
    @Html.DropDownListFor(m=>m.id, Model.DDLCollecton)

    <input type="text" name="SearchString" />
    <input type="submit" value="search" />
}

<div id="divList">
    @Html.Partial("_EmployeeList", Model.Employees);
</div>

<script type="text/javascript">
    function updateDivList() {
        $('#divList').html();
    }

</script>

控制器：

public class EmployeeController : Controller
{
    //
    // GET: /Employee/
    testdbEntities db = new testdbEntities();

    public ActionResult Index()
    {
        List<SelectListItem> ddlColl = new List<SelectListItem> 
        { 
            new SelectListItem{ Text="By Emp No", Value="1", Selected=true}, 
            new SelectListItem{ Text="By Name", Value="2"}
        };

        var employees = db.Employees.Select(e => new EmployeeModel 
        {
            EmpID=e.EmpId, 
            FirstName=e.FirstName, 
            LastName=e.LastName, 
            DeptId=e.DepartmentId, 
            DepartmentName=e.Department.Name
        });

        EmployeeViewModel evm = new EmployeeViewModel { DDLCollecton = ddlColl, Employees = employees };

        return View(evm);
    }


    public ActionResult GetEmployee(string id, string SearchString)
    {
        IEnumerable<EmployeeModel> results=null;

        if (id == "1")
        {

            var searchID = ( SearchString == "" ) ? 0 : int.Parse(SearchString);

            results = db.Employees.Where( e => ( SearchString == "" || e.EmpId == searchID ) )
                .Select(e => new EmployeeModel
                {
                    EmpID = e.EmpId,
                    FirstName = e.FirstName,
                    LastName = e.LastName,
                    DeptId = e.DepartmentId,
                    DepartmentName = e.Department.Name
                }).ToList();
        }   
        else
        { 

        }

        return PartialView("_EmployeeList", results);
    }

当我请求ajax时，它会将请求作为非ajax调用发送，但会将数据作为常规http请求获取，如this video

所示

为什么会发生这种情况？如何解决这个问题？