radiobuttonlist selectedindexchanged事件每次都不会触发

Question

这是数据绑定代码：

    protected void Page_Init(object sender, EventArgs e)
    {
        List<ListItem> source = new List<ListItem>();
        source.Add(new ListItem("Remark A", "0"));
        source.Add(new ListItem("Remark B", "1"));
        source.Add(new ListItem("Remark C", "2"));

        rbnRemarks.DataSource = source;
        rbnRemarks.DataBind();
    }
    protected void rbnRemarks_SelectedIndexChanged(object sender, EventArgs e)
    {
        refreshGrid();
    }

这是标记页面：

    <asp:RadioButtonList OnSelectedIndexChanged="rbnRemarks_SelectedIndexChanged" AutoPostBack="true" runat="server" ID="rbnRemarks" RepeatDirection="Horizontal">
    </asp:RadioButtonList>

问题是：

如果我将所选项目更改为备注B或备注C，则会触发事件，但如果我将所选项目更改回备注A，则不会触发事件并且仅当我选择备注B或备注时才会触发Page_Init C，为什么会发生这种情况？