为什么SelectedIndexChanged事件不是从GridView中的下拉列表触发的？

<asp:GridView ID="GridView1" runat="server">
   <Columns>
        <asp:TemplateField>
            <ItemTemplate>
                <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
                    <asp:ListItem>Item 1</asp:ListItem>
                    <asp:ListItem>Item 2</asp:ListItem>
                    <asp:ListItem>Item 3</asp:ListItem>
                </asp:DropDownList>
            </ItemTemplate>
        </asp:TemplateField>
    </Columns>
</asp:GridView>