PopupControlExtender for checkbox list. checked status retained even after clearing the checkbox list

Question

I am using PopupControlExtender which pops up a panel. Inside the panel I have a CheckBoxList Everything is inside an update panel. Now if I select some items from the check box and use this in a method it's working fine. But for any successive save it retaining the true value of checked property of any previously selected item. I tried to clear the item as well.

bellow is the asp code.

<ajaxtoolkit:PopupControlExtender Position="Bottom" CommitProperty="value" CommitScript="e.value='Selected';"  runat="server" ID="ddeAssignedTo"  TargetControlID="lblAssignedTo" PopUpControlID="DropPanelAssignedTo" />
<asp:TextBox ID="lblAssignedTo" ReadOnly="true" runat="server" Text="Select Users"></asp:TextBox>
<asp:Panel ID="DropPanelAssignedTo" CssClass="dropPanelArea" runat="server">
   <asp:CheckBoxList ID="chklstAssignTo"  runat="server"></asp:CheckBoxList>
</asp:Panel>

I am getting true for a listitem's Selected property, which was not actually checked before submitting the page, but was checked in previous submit.

In fact the list item checked retains the value even after executing bellow code

chklstAssignTo.ClearSelection();
chklstAssignTo.Items.Clear();

Please help me out