网格视图单元格在分类汇总网格视图的底部自动添加 0

为什么网格视图单元格会自动在分类汇总网格视图的底部添加 0？

        int qty = 0;
        int amount = 0;
        int rowtotal = 0;
        int subtotal = 0;
        int total = 0;
        foreach (DataGridViewRow row in itemgridview.Rows)
        {
            qty = Convert.ToInt32(row.Cells[1].Value); 
            amount = Convert.ToInt32(row.Cells[2].Value); 
            rowtotal = qty * amount;
            row.Cells[3].Value = rowtotal;
            subtotal += rowtotal;
        }
        total = subtotal;
        txtordertotal.Text = total.ToString();