TextFormat weirdness

I have the following code:

package {

    import flash.display.Sprite;
    import flash.events.TimerEvent;
    import flash.text.TextField;
    import flash.text.TextFieldAutoSize;
    import flash.text.TextFormat;
    import flash.utils.Timer;
    public class AnimateText {
        private var _text:String;
        private var _holder:Sprite;
        private var _textfield:TextField;
        private var _fmt:TextFormat;
        private var _timer:Timer;
        private var _cont:uint = 1;
        public function AnimateText() {
            _text = "Hello world";
            _holder = new Sprite();
            _fmt = new TextFormat();
            _fmt.font = "Arial";
            _fmt.size = 18;
            _textfield = new TextField();
            _textfield.autoSize = TextFieldAutoSize.LEFT;
            _timer = new Timer(100, _text.length);
            _timer.addEventListener(TimerEvent.TIMER, onTimerTick);
        private function onTimerTick(evt:TimerEvent):void {
            _textfield.text = _text.substr(0, _cont++);

When publishing, the textfield does not have any textformat. However, if I change the following:

private function onTimerTick(evt:TimerEvent):void {
            _textfield.text = _text.substr(0, _cont++);

It works fine. Why do I have to apply the TextFormat in every tick of the Timer? It does the same with an ENTER_FRAME event, and I don’t understand it.