GridView1中的textbox中的数字无法分别写入数据库，而且只能把最后一个textbox写入，其余的都和最后一个的数字相同，这是为什么？

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.IO;
using System.Data.SqlClient;

public partial class T_Scoring : System.Web.UI.Page
{
  protected void Page_Load(object sender, EventArgs e)
  {
  if (!IsPostBack)
  {
  this.bind();
  }
  }
  public void bind()
  {
  string sqlstr = "select * from UpRrport";
  SqlConnection sqlcon = new SqlConnection();
  sqlcon.ConnectionString = ConfigurationManager.ConnectionStrings["ConnectionString5"].ConnectionString;
  sqlcon.Open();
  SqlDataAdapter myda = new SqlDataAdapter(sqlstr, sqlcon);
  DataSet myds = new DataSet();
  myda.Fill(myds, "UpRrport");
  GridView1.DataSource = myds;
  GridView1.DataKeyNames = new string[] { "ID" };

  GridView1.DataBind();
  sqlcon.Close();
  }
  protected void Button1_Click(object sender, EventArgs e)
  {
  for (int i = 0; i <= GridView1.Rows.Count - 1; i++)
  {
  string myAchievement = ((TextBox)GridView1.Rows[i].FindControl("TextBox2")).Text;
  SqlConnection myCon = new SqlConnection();
  myCon.ConnectionString = ConfigurationManager.ConnectionStrings["ConnectionString5"].ConnectionString;
  myCon.Open();


  //2 实例化一个SqlCommand对象
  SqlCommand myCom = new SqlCommand();

  //3 利用修改命令赋值SqlCommand的CommandText
  myCom.Connection = myCon;
  myCom.CommandText = "Update UpRrport set achievement='" + myAchievement + "'";
  //4 执行增加记录
  myCom.ExecuteNonQuery();

  //5 断开连接
  myCon.Close();
  }
  Response.Write("<script>alert('修改成功');location.href('T-Scoring.aspx')</script>");
  }
  
}

debug调试一下 估计循环那有问题