I am transferring data from one GridView to another GridView, after transferring data then GridView is not displaying correctly Column sum.

Decimal DDQTY = 0;
Decimal DDWeight = 0;
protected void GVballist_RowDataBound(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.DataRow)
    {
        Label lblDQ = (Label)e.Row.FindControl("Qty");
        DDQTY += Convert.ToDecimal(lblDQ.Text);
        Label lblDWT = (Label)e.Row.FindControl("BWeight");
        DDWeight += Convert.ToDecimal(lblDWT.Text);
    }
    else if (e.Row.RowType == DataControlRowType.Footer)
    {
        Label lbl = (Label)e.Row.FindControl("lblQTotal");
        lbl.Text = DDQTY.ToString();
        Label lbl1 = (Label)e.Row.FindControl("lblWTotal");
        lbl1.Text = DDWeight.ToString();
    }
}

when I am selecting multiple row with checkbox in GridView, then update database table with a ID. 

after updating table in database, then query executed and fetch data from database again into GridView, then it is not calculating column sum properly.

protected void btn_Trfer_Click(object sender, EventArgs e)
{
    if (lbinvid.Text == "")
    {
        Response.Write("<script>alert('Please Generate Invoice ID.')</script>");
    }
    else
    {
        foreach (GridViewRow row in GVballist.Rows)
        {
            if ((row.FindControl("CheckBox1") as CheckBox).Checked)
            {
                int id = Convert.ToInt32((row.FindControl("hfBID1") as HiddenField).Value);

                using (SqlCommand cmd = new SqlCommand("Sp_Ret_DisBale_BB_INv", con))
                {
                    cmd.Parameters.AddWithValue("@Action", "Update_Invoice");
                    cmd.CommandType = CommandType.StoredProcedure;
                    cmd.Parameters.AddWithValue("@ID", id);
                    cmd.Parameters.AddWithValue("@Inv_ID", lbinvid.Text);
                    con.Open();
                    cmd.ExecuteNonQuery();
                    con.Close();
                    lbmsg.Text = "Transfer Successfully.";
                    lopadgvtranfer();
                }
            }
        }
    }