How does Unity null all references to a GameObject after Object.Destroy is called on it?

test.cs

using UnityEngine;

public class Main : MonoBehaviour {
	void Start ()
	{
		FakeObject obj = new FakeObject ();
		Debug.LogError (obj); // [FakeObject]
		Debug.LogError (obj == null); // false
		Debug.LogError (obj.num); //100

		FakeObject.DestroyImmediate (obj); 

		Debug.LogError (obj); // null
		Debug.LogError (obj == null);//true
		Debug.LogError (obj.num); //100
	}
}


class FakeObject
{
	public int num = 100;
	private bool destroyed = false;
	public static void DestroyImmediate(FakeObject obj)
	{
		obj.destroyed = true;
	}

	public override string ToString ()
	{
		if (destroyed) 
		{
			return "null";
		}
		return string.Format ("[FakeObject]");
	}
	
	private static bool CompareBaseObjects (FakeObject x, FakeObject y)
	{
		if ((object)x != null && x.destroyed) {
			x = null;
		}
		if ((object)y != null && y.destroyed) {
			y = null;
		}
		return object.Equals (x, y);
	}

	public static bool operator == (FakeObject x, FakeObject y)
	{
		return CompareBaseObjects (x, y);
	}

	public static bool operator != (FakeObject x, FakeObject y)
	{
		return !CompareBaseObjects (x, y);
	}
}