A Binding Formatter is Called Twice

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
using System.Diagnostics;

namespace SampleWindowsApp
{
    /// <summary>
    /// Summary description for Form1.
    /// </summary>
    public class Form1 : System.Windows.Forms.Form
    {
        private System.Windows.Forms.TextBox textBox1;
        private System.Windows.Forms.Button button1;
        private MyClass _dataSource = new MyClass();

        /// <summary>
        /// Required designer variable.
        /// </summary>
        private System.ComponentModel.Container components = null;

        public Form1()
        {
            //
            // Required for Windows Form Designer support
            //
            InitializeComponent();
        }

        /// <summary>
        /// Clean up any resources being used.
        /// </summary>
        protected override void Dispose( bool disposing )
        {
            if( disposing )
            {
                if (components != null)
                {
                    components.Dispose();
                }
            }
            base.Dispose( disposing );
        }

        #region Windows Form Designer generated code
        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InitializeComponent()
        {
            this.textBox1 = new System.Windows.Forms.TextBox();
            this.button1 = new System.Windows.Forms.Button();
            this.SuspendLayout();
            //
            // textBox1
            //
            this.textBox1.Location = new System.Drawing.Point(56, 32);
            this.textBox1.Name = "textBox1";
            this.textBox1.TabIndex = 0;
            this.textBox1.Text = "textBox1";
            this.textBox1.Leave += new System.EventHandler(this.textBox1_Leave);
            //
            // button1
            //
            this.button1.Location = new System.Drawing.Point(72, 120);
            this.button1.Name = "button1";
            this.button1.TabIndex = 1;
            this.button1.Text = "button1";
            //
            // Form1
            //
            this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
            this.ClientSize = new System.Drawing.Size(292, 266);
            this.Controls.Add(this.button1);
            this.Controls.Add(this.textBox1);
            this.Name = "Form1";
            this.Text = "Form1";
            this.Load += new System.EventHandler(this.Form1_Load);
            this.ResumeLayout(false);

        }
        #endregion

        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.Run(new Form1());
        }

        private void Form1_Load(object sender, System.EventArgs e)
        {
            Binding binding = new Binding( "Text", _dataSource, "Value" );
            binding.Format += new ConvertEventHandler(binding_Format);

            textBox1.DataBindings.Add( binding );
        }

        private void binding_Format(object sender, ConvertEventArgs e)
        {
            Debug.WriteLine( "Format" );
        }

        private void textBox1_Leave(object sender, System.EventArgs e)
        {
            Debug.WriteLine( "Leave" );

        }
    }

    class MyClass
    {
        private string _value = "Default value.";
        public string Value
        {
            get
            {
                Debug.WriteLine( "MyClass.Value getter" );
                return _value;
            }
            set
            {
                Debug.WriteLine( "MyClass.Value setter" );
                _value = value;
            }
        }
    }
}