Unbelievable! a Labels created dynamically invisible just because they overlapped, buttons - OK, why is that?

Question

some controls have same behavior (like Label) some like button.
To test it: create WindowsApplication, put a button (name=button1) in the right lower corner
and replace your Form1.cs with:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
/* What is amazing!!! among all created labels visible is first
* just because they overlap a little bit. Make it not a label but a button
* commenting line designated in oneLabel method as // 1 and uncomment // 2
* and you will be able to see all created buttons still overlapped
* why is that?
*/

namespace WindowsApplication1
{
   public partial class Form1 : Form
   {
      private int counter,locY = 0;
      public Form1()
      {
          InitializeComponent();
      }
      private void oneLabel()
      {
           Label la = new Label(); // 1
           //Button la = new Button(); // 2
           la.Text = "la_" + counter;
           la.Top = locY+10;
           la.Left = 110;
           this.Controls.Add(la);
           counter++;
      }
      private void button1_Click(object sender, EventArgs e)
      {
           oneLabel();
           locY += 10; // if you specify 30 instead of 10 they will become visible!!!
      }
    }
}