Why HiddenField in UserControl get the same Id

Question

In Investor.aspx page, the DataList1 is binded. The DataList1 has UserControl that get parameter "DistId" which is used for futher when I click on hlOpenDoc.
Because I can't set "onclick" attribute to HyperLink, that would be procceeded on server, I created hidden button "btnSetEvent " that is clicked from javascript.
At this moment "btnSetEvent" is vissible only for the purpose of understanding my porblm.
The problem:
When I click on hlOpenDoc(that is in UserControl) I noticed that hfDistID has the same DistId regardless on which row I click the button 
hlOpenDoc.
But when I click on "btnSetEvent" the hfDistID control has correct value that depends on which row I clicked.
Why it is happens?
Thank you.

Investor.aspx
<asp:DataList ID="DataList1" runat="server" CellSpacing="4" Width="545px" DataSourceID="SqlHanpakot">
    <ItemStyle BackColor="LightGray" BorderColor="Transparent" Height="20px" />
    <SeparatorStyle BackColor="Transparent" BorderColor="Transparent" />
    <ItemTemplate>
        <table style="width: 100%">
            <tr>                
                <td style="width: 55px; height: 38px; background-position: center center; text-align: center;">                   
                    <uc:OpenDoc DistID='<%# Eval("DistId") %>' NavigateUrl='<%# Eval("sDocPath")+"#pagemode=thumbs" %>' ID="OpenDoc1" runat="server"/>                                                        
                </td>
            </tr>
        </table>       
    </ItemTemplate>
</asp:DataList><

user control(OpenDoc.ascx)

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="OpenDoc.ascx.cs" Inherits="OpenDoc" %>
<script type="text/javascript">
    function setEvent() {
        document.getElementsByName('<%= btnSetEvent.UniqueID %>')[0].click();
    }
</script>
<asp:HyperLink ID="hlOpenDoc" runat="server"
                              Font-Names="Arial"
                              Font-Size="10pt"
                              Font-Underline="False"
                              ForeColor="Green"
                              Width="93px"
                              Target="_blank" 
                              onclick="setEvent();">Open document</asp:HyperLink>

<asp:HiddenField ID="hfDistID" runat="server" />
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
    <ContentTemplate>
        <asp:Button ID="btnSetEvent" runat="server"
                                     Text="Button"                                      
                                     onclick="btnSetEvent_Click"/>
    </ContentTemplate>
</asp:UpdatePanel>

code behind(OpenDoc.ascx.cs)

public partial class OpenDoc : System.Web.UI.UserControl
{
    public string NavigateUrl
    {        
        set
        {
            hlOpenDoc.NavigateUrl = value;
        }
    }
  
    public string DistID
    {
        set
        {
            hfDistID.Value = value;
        }
    }    
    
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void btnSetEvent_Click(object sender, EventArgs e)
    {

    }
}