Why there are different ways to set label text on Qt Installer Framework?

Question

With Qt Installer Controller Script I see how to change the text of some labels on the Introduction page. I did

function Controller()
{
}

Controller.prototype.IntroductionPageCallback = function()
{

    var widget = gui.currentPageWidget();
    if (widget != null) {
        widget.title = "New title"; 
        widget.MessageLabel.setText("New Message."); 
    }
}

Why they work differently? To set title I do = "New title" but to set MessageLabel I do .setText("New Message.").