Why all the page in the GridView show the same data as that in the first page?

See more:

C#

ASP.NET

The HTML code of the GrdiView is:
-----------------------------------------------------------------------------------

<asp:GridView ID="GridView_NewStaff" runat="server" AutoGenerateColumns="False" 
 CellPadding="4" Font-Size="Small" ForeColor="#333333" AllowPaging="True" 
 AllowSorting="True" onpageindexchanging="GridView_NewStaff_PageIndexChanging" 
 PageSize="20">
 <pagersettings firstpagetext="First" lastpagetext="Last">
     Mode="NextPreviousFirstLast" NextPageText="Next" PageButtonCount="4" 
     PreviousPageText="Previous" />
	 <columns>......

-----------------------------------------------------------------------------------

The C# code is:
-----------------------------------------------------------------------------------

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        bindGridView();
    }
}

protected void Button_ReadFile_Click(object sender, EventArgs e)
{
	bindGridView();
}


public void bindGridView()
{
    if (FileUpload_StaffInfo.HasFile)
    {
        string InfoFile_Name = Server.MapPath(InfoFileDirectory) + FileUpload_StaffInfo.FileName;
        FileUpload_StaffInfo.SaveAs(InfoFile_Name);

        dataTable_Excel = new DataTable();

        if (InfoFile_Name.EndsWith(".xlsx"))//Read Excel2007-2010 format file
        {
            StaffExcelXML StaffNewXML = new StaffExcelXML();
	    StaffNewXML.ExcelInput(InfoFile_Name, ref dataTable_Excel)
        }
        else if (InfoFile_Name.EndsWith(".xls"))//Read Excel1997-2003 format file
        {
            StaffXLS StaffNewXLS = new StaffXLS();
	    StaffNewXLS.ExcelInput(InfoFile_Name, ref dataTable_Excel);
        }

        GridView_NewStaff.DataSource = dataTable_Excel;
        GridView_NewStaff.DataBind();
    }
}

-----------------------------------------------------------------------------------
Everytime after clicking the Next, or the Previous, or the First, or the Last, it always shows the first page's data. Could anyone help me with this?