concurrenthashmap - フェイル セーフ フェイル ファストの例

Question

    ConcurrentHashMap<String, Integer> hm = new ConcurrentHashMap<>();
    hm.put("1", 1);
    hm.put("2", 2);
    hm.put("3", 3);
    Iterator<String> itr = hm.keySet().iterator();
    while(itr.hasNext()){
        String key = itr.next();
        System.out.println(key + " : " + hm.get(key));
        hm.put("4", 4);
    }
    System.out.println(hm);

    ConcurrentHashMap<String, Integer> hm1 = new ConcurrentHashMap<>();
    hm1.put("One", 1);
    hm1.put("Two", 2);
    hm1.put("Three", 3);
    Iterator<String> itr1 = hm1.keySet().iterator();
    while(itr1.hasNext()){
        String key = itr1.next();
        System.out.println(key + " : " + hm1.get(key));
        hm1.put("Four", 4);
    }
    System.out.println(hm1);

出力: 1 : 1 2 : 2 3 : 3 4 : 4 {1=1, 2=2, 3=3, 4=4} One : 1 Two : 2 Three : 3 {One=1, Four=4, Two =2、3=3}

なんでそうなの？