关于Atomicboolean在并发访问完成时的奇怪行为的问题

我得到了这样的代码：

public class ConcurrencyCheck {
    private volatile static AtomicBoolean top=new AtomicBoolean(false);
    private  int i=0;

    public class Toppler extends Thread{
        private final boolean bool;

        public Toppler(boolean myBool,String name) {
            super(name);
            bool=myBool;
        }

        @Override
        public void run() {
            while(!isInterrupted()){

                i++;
                synchronized (top) {
                    if (top.get() == bool) top.set(!top.get());
                    System.err.println(super.getName() + "  "+ bool +"->" + top + ". i is " + i);
                }

                try {
                    sleep(100);
                } catch (InterruptedException e) {
                    break;
                }
            }
        }
    }

    public static void main(String[] args) throws InterruptedException {
        ConcurrencyCheck cc = new ConcurrencyCheck();
        Thread t1= cc.new Toppler(true,"thread1");
        Thread t2= cc.new Toppler(false,"thread2");
        Thread t3= cc.new Toppler(true,"thread3");
        Thread t4= cc.new Toppler(false,"thread4");
        Thread t5= cc.new Toppler(true,"thread5");
        Thread t6= cc.new Toppler(false,"thread6");
        Thread t7= cc.new Toppler(true,"thread7");
        Thread t8= cc.new Toppler(false,"thread8");
        t1.start();
        ...
        t8.start();
        sleep(950);
        t1.interrupt();
        ...
        t8.interrupt();
    }
}

它旨在检查 AtomicBoolean 的工作方式。Toppler 类是一个线程，它会定期推翻 Atomic 布尔值。推翻布尔值的代码块是同步的。正如我猜测的那样，每个输出行都必须推翻“top”变量的值，因此输出必须是“true->false false->true true->false...”。但出于某种原因，有时我可以看到这样的输出：

thread1  true->false. i is 1
thread8  false->true. i is 8
thread7  true->false. i is 8
thread4  false->true. i is 8
thread6  false->true. i is 8
thread3  true->false. i is 8
thread5  true->false. i is 8
thread2  false->true. i is 8
thread1  true->false. i is 9
thread8  false->true. i is 10
thread7  true->false. i is 11
thread4  false->true. i is 12
thread6  false->true. i is 13
thread3  true->false. i is 14

问题是：为什么可能出现两个随后的 false->true 或 true->false 输出行？