我做了个用MasterPage作主页面的程序，分左中右三部分，左侧25%，为什么这部分会根据右侧两个的宽度改变而改变呢？？

<tr id="ContentTr" valign="top" height="75%">
    <td id="LeftTd" width="25%" valign="top"style="border-left: #ffffff thick dotted;">
                    <asp:ContentPlaceHolder ID="LeftContent" runat="server" Visible="False">
                     </asp:ContentPlaceHolder>
    </td>
    <td width="*" id="CenterTd" valign="top" style="border-left: gainsboro thin dotted;">
                    <asp:ContentPlaceHolder ID="CenterContent" runat="server" Visible="False">
                    </asp:ContentPlaceHolder>
    </td>
    <td width="*" id="RightTd" valign="top" style="border-left: antiquewhite thin dotted;">
                    <asp:ContentPlaceHolder ID="RightContent" runat="server" Visible="False">
                    </asp:ContentPlaceHolder>
    </td>
</tr>
[em18][em18][em18]