Dúvida Thread - Livro Kathy Sierra

11. Given:

class ThreadDemo {
	synchronized void a() {
		actBusy();
	}

	static synchronized void b() {
		actBusy();
	}

	static void actBusy() {
		try {
			Thread.sleep(1000);
		} catch (InterruptedException e) {
		}
	}

	public static void main(String[] args) {
		final ThreadDemo x = new ThreadDemo();
		final ThreadDemo y = new ThreadDemo();
		Runnable runnable = new Runnable() {
			public void run() {
				int option = (int) (Math.random() * 4);
				switch (option) {
				case 0:
					x.a();
					break;
				case 1:
					x.b();
					break;
				case 2:
					y.a();
					break;
				case 3:
					y.b();
					break;
				}
			}
		};
		Thread thread1 = new Thread(runnable);
		Thread thread2 = new Thread(runnable);
		thread1.start();
		thread2.start();
	}
}

Which of the following pairs of method invocations could NEVER be executing at the same time? (Choose all that apply.) A. x.a() in thread1, and x.a() in thread2 B. x.a() in thread1, and x.b() in thread2 C. x.a() in thread1, and y.a() in thread2 D. x.a() in thread1, and y.b() in thread2 E. x.b() in thread1, and x.a() in thread2 F. x.b() in thread1, and x.b() in thread2 G. x.b() in thread1, and y.a() in thread2 H. x.b() in thread1, and y.b() in thread2

Answer:
✓ A, F, and H. A is a right answer because when synchronized instance methods are called
on the same instance, they block each other. F and H can't happen because synchronized
static methods in the same class block each other, regardless of which instance was used
to call the methods. (An instance is not required to call static methods; only the class.)

X C could happen because synchronized instance methods called on different instances
do not block each other. B, D, E, and G could all happen because instance methods and
static methods lock on different objects, and do not block each other. (Objective 4.3)

Não entendi o porquê de acessos simultâneos bloquearem um ao outro, através do que estudei, quando isso
acontecia elas deveriam fazer fila conforme liberação do lock.

Talvez tenha entendido erro, alguém pode me ajudar ?

Obrigado !