Zend_Form SubForm输入名称不同

I followed the tutorial here: http://framework.zend.com/manual/1.12/en/zend.form.advanced.html on creating a multi-page form in Zend Framework. However, I'm displaying the form slightly differently and it's led to unexpected behavior. If I display the form like this:

<?php echo $this->form; ?>

I end up with the expected input with name="user[username]":

<dt id="user-username-label">
  <label for="user-username" class="required">Username:</label></dt>
<dd id="user-username-element">
  <input type="text" name="user[username]" id="user-username" value="" /></dd>

However, if I want a little more control over how the individual form elements are displayed:

<?php echo $this->form->username; ?>

I get name="username":

<dt id="username-label">
  <label for="username" class="required">Username:</label></dt>
<dd id="username-element">
  <input type="text" name="username" id="username" value="" /></dd>

What's interesting is that the submit button gets the correct name="user[submit]" either way. What gives? Is there something Zend_Form::__toString() or Zend_Form::render() that I'm missing?