Winform代码、设计器添加控件 顶层底层位置不同

设计器里添加时先加的控件在底层，后拖拽过去的控件覆盖在之前控件上（在顶层）；

代码添加时，先加的控件在顶层，后加的反而在底层，为什么？

        private void Form1_Load(object sender, EventArgs e)
        {
            TextBox pa1 = new TextBox();
            pa1.Location = new Point(110, 110);
            pa1.Size = new Size(30, 30);
            pa1.BackColor = Color.Black;
            this.Controls.Add(pa1);

            Panel pa = new Panel();
            pa.Location = new Point(100, 100);
            pa.Size = new Size(100, 100);
            pa.BackColor = Color.White;
            this.Controls.Add(pa);            
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            

            Panel pa = new Panel();
            pa.Location = new Point(100, 100);
            pa.Size = new Size(100, 100);
            pa.BackColor = Color.White;
            this.Controls.Add(pa);

            TextBox pa1 = new TextBox();
            pa1.Location = new Point(110, 110);
            pa1.Size = new Size(30, 30);
            pa1.BackColor = Color.Black;
            this.Controls.Add(pa1);
        }