myTextView.setText () can be performed in a non-UI thread, Why

Hello, I have a question on the use of Schedulers in the process, the operation of the UI in Android development, should be the main thread of execution, why I use the observeOn (Schedulers.io ()), TextView can still assignment, which is how to do, please help me to answer, very grateful.Code is as follows:

myTextView.setText("thread 1 id=" + Thread.currentThread().getId());
Observable.just(1, 2, 3, 4)
    .subscribeOn(Schedulers.io())
    .observeOn(Schedulers.newThread())
    .map(new Func1<Integer, String>() {
        @Override
        public String call(Integer integer) {
            myTextView.setText(myTextView.getText() + "\n" + "thread 2 id =" +
                 Thread.currentThread().getId());
            return "5" + integer;
        }
    })
    .observeOn(Schedulers.io())
    .map(new Func1<String, Integer>() {
        @Override
        public Integer call(String s) {
            myTextView.setText(myTextView.getText() + "\n" + "thread 3 id =" + 
                  Thread.currentThread().getId());
            return Integer.parseInt(s);
        }
    });

Results ： TextView normal display

[akarnokd]

You didn’t schedule on the proper thread. You have to use observeOn(AndroidSchedulers.mainThread()) so setText runs on the main thread.

[EhsanMashhadi]

Any clue about the reason why it is work on onCreate method?