Gridview的表头固定后，怎么会产生遮挡？请各位大侠指教。

把Gridview的表头固定后，对GMDatePicker空间的日历列表产生了遮挡效果，而且只有表头header遮挡，gridview的其它部位正常，这是什么原因造成的？请各位大侠，多多指教。
Gridview前台代码：
<div  class="gridviewc" id="dvBody">       
        <asp:GridView ID="Grid_SearchResult" runat="server" AutoGenerateColumns="False" 
            BackColor="White" BorderColor="#CCCCCC" BorderStyle="None" BorderWidth="1px" 
            CellPadding="3" Font-Overline="False" Font-Size="Small" Font-Strikeout="False">
            <FooterStyle BackColor="White" ForeColor="#000066" />
            <RowStyle ForeColor="#000066" />
            <Columns>
                <asp:BoundField HeaderText="单位" DataField="sgdw" />
                <asp:BoundField HeaderText="姓名" DataField="xm" />
                <asp:BoundField HeaderText="日期" DataField="rq" DataFormatString="{0:yyyy-MM-dd}" HtmlEncode="false"/>
                <asp:BoundField HeaderText="班次" DataField="bc" />
                <asp:BoundField HeaderText="积分性质" DataField="jfxz" />
                <asp:BoundField HeaderText="积分原因" DataField="jfyy" ItemStyle-Width="200px"/>
                <asp:BoundField HeaderText="积分值" DataField="jfz" />               
            </Columns>
            <PagerStyle BackColor="White" ForeColor="#000066" HorizontalAlign="Left" />
            <SelectedRowStyle BackColor="#669999" Font-Bold="True" ForeColor="White" />
            <HeaderStyle BackColor="#006699" Font-Bold="True" ForeColor="White" CssClass="Freezing" />
        </asp:GridView> 
       </div>
Gridview表头固定样式：
/*Gridview  cha xun bang ding yang shi*/
.Freezing 
{ 
position:relative ; 
table-layout:fixed;
top:expression(this.offsetParent.scrollTop); 
z-index: 10;
} 
.Freezing th{text-overflow:ellipsis;overflow:hidden;white-space: nowrap;padding:2px;}
.gridviewc
{
   overflow-y: scroll;
   height: 300px;
   width:600px;
   
    
}GMDDatePicker样式引用：
<cc1:GMDatePicker ID="DatePicker" runat="server" CalendarFont-Names="Arial" 
            Width="60px" CssClass="textbox"> 
<CalendarDayStyle Font-Size="9pt" />
<CalendarTodayDayStyle BorderWidth="1" BorderColor="darkred" Font-Bold="true" />
<CalendarFont Names="Arial"></CalendarFont> <CalendarOtherMonthDayStyle BackColor="whitesmoke" />
<CalendarTitleStyle BackColor="#E0E0E0" Font-Names="Arial" Font-Size="9pt" />
</cc1:GMDatePicker>