gridview宽度设定问题

在gridview中如果设定了列的宽度，为什么在绑定的时候宽度还会改变
如ItemStyle-Width="60px" HeaderStyle-Width="60px" 
实际的HTML 宽度不会有60px 只有47
gridiview代码
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  <asp:GridView ID="gv_Main" runat="server" AutoGenerateColumns="False"  
                  Width="670px" ShowHeader="False">
              <Columns>
                  <asp:BoundField DataField="itemSort" HeaderText="排序" ItemStyle-Width="60px" HeaderStyle-Width="60px" >
                   </asp:BoundField>
                  <asp:BoundField DataField="itemName" HeaderText="项目名称" ItemStyle-Width="150px" HeaderStyle-Width="150px">
                   </asp:BoundField>
                  <asp:TemplateField HeaderText="是否为工资总额" ItemStyle-Width="80px" HeaderStyle-Width="80px">
                      <ItemTemplate>
                          <asp:Label ID="lab_ToTal" runat="server" Width="80px" Text='<%#GridViewDataChange(Eval("toTal").ToString()) %>'></asp:Label>
                      </ItemTemplate>
                  </asp:TemplateField>
                  <asp:TemplateField HeaderText="是否为应税工资总额" ItemStyle-Width="80px" HeaderStyle-Width="80px">
                    <ItemTemplate>
                          <asp:Label ID="lab_PreTax" runat="server" Width="80px" Text='<%#GridViewDataChange(Eval("preTax").ToString()) %>'></asp:Label>
                      </ItemTemplate>
                  </asp:TemplateField>
                  <asp:TemplateField HeaderText="是否为应扣税额" ItemStyle-Width="80px" HeaderStyle-Width="80px">
                    <ItemTemplate>
                          <asp:Label ID="lab_AfterTax" runat="server" Width="80px" Text='<%#GridViewDataChange(Eval("afterTax").ToString()) %>'></asp:Label>
                      </ItemTemplate>
                  </asp:TemplateField>
                  <asp:TemplateField HeaderText="月结项目清零" ItemStyle-Width="80px" HeaderStyle-Width="80px">
                    <ItemTemplate>
                          <asp:Label ID="lab_Reset" runat="server" Width="80px" Text='<%#GridViewDataChange(Eval("reset").ToString()) %>'></asp:Label>
                      </ItemTemplate>
                  </asp:TemplateField>
                  <asp:TemplateField HeaderText="是否启用" ItemStyle-Width="80px" HeaderStyle-Width="80px">
                    <ItemTemplate>
                          <asp:Label ID="lab_Enabled" runat="server" Width="80px" Text='<%#GridViewDataChange(Eval("enabled").ToString()) %>'></asp:Label>
                      </ItemTemplate>
                  </asp:TemplateField>
                   <asp:TemplateField HeaderText="操作" ItemStyle-Width="60px" HeaderStyle-Width="60px">
                       <ItemTemplate>
                           <input ID="btn_Update" type="button" Width="60px"   class="btneditright" value="修改" onclick=' showDiv("Update","<%#Eval("salaryItemId") %>");'  />
                       </ItemTemplate>
                   </asp:TemplateField>
              </Columns>
              <EmptyDataTemplate>
              不存在工资项目！
              </EmptyDataTemplate>
              <HeaderStyle CssClass="THEAD" />
          </asp:GridView>