Why HiddenField in UserControl get the same Id

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
"DataList1" runat="server" CellSpacing="4" Width="545px" DataSourceID="SqlHanpakot">
    "LightGray" BorderColor="Transparent" Height="20px" />
    "Transparent" BorderColor="Transparent" />
    
        "width: 100%">



            
                                
"width: 55px; height: 38px; background-position: center center; text-align: center;">                   
                    "OpenDoc1" runat="server"/>                                                        
                
            
        
       
    
<

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="OpenDoc.ascx.cs" Inherits="OpenDoc" %>

"hlOpenDoc" runat="server"
                              Font-Names="Arial"
                              Font-Size="10pt"
                              Font-Underline="False"
                              ForeColor="Green"
                              Width="93px"
                              Target="_blank" 
                              onclick="setEvent();">Open document

"hfDistID" runat="server" />
"UpdatePanel1" runat="server">
    
        "btnSetEvent" runat="server"
                                     Text="Button"                                      
                                     onclick="btnSetEvent_Click"/>
    

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)
    {

    }
}