为什么用ASP.NET打印九九乘法表不是乘法表的形式呢

txtStart是开始值的文本框名称，txtEnd是结束值的文本框名称，如果在开始值写1，结束值写9，程序运行出来，就是9*9=81，不是乘法表的形式，不知道为什么
using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class _9_9 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void btnSubmit_Click(object sender, EventArgs e)
    {
        if (Convert.ToInt32(txtStart.Text.ToString())<1||(Convert.ToInt32(txtEnd.Text.ToString())>9))
        {
            lblMessage.Text = "对不起，您输入的值不符合";
        }
        else
        {
            for (int i = Convert.ToInt32(txtStart.Text.Trim()); i <=Convert.ToInt32(txtEnd.Text.Trim()); i++)
            {
                for (int j = 1; j <=i; j++)
                {
                    lblMessage.Text = i + "*" + j + "=" + i * j;

                }
            }
        }
    }
}
希望高手们帮我看看，谢谢