Comment style changed automatically

Recently I just realized my comment layout has changed automatically. It used to be:

User name field
User email address field
User website url field
Comment field
Submit button

But then without doing anything to comments.php and style.css under my theme folder, the fields swap by themselves and turn into:

Comment field
User name field
User email address field
User website url field
Submit button

I don’t know when this has effected. I check the comments.php and it shows:

<?php
$comment_args = array(

		'fields' => apply_filters(
  		'comment_form_default_fields', array(
			'author' => '<p class="comment-form-author">' ...
			'email'  => '<p class="comment-form-email">' ...
			'url'	 => '<p class="comment-form-url">' ...
  			)
  		),
		'comment_notes_before' => '',
		'comment_notes_after'  => '',
		'title_reply'          => __( 'Put your comment here' ),
		'title_reply_to'       => __( 'Reply to %s' ),
		'label_submit'         => __( 'Post' ),
		'comment_field'        => '<p class="comment-form-comment"><label for="comment">' ...
    );
 comment_form($comment_args);
?>

Based on the sequence it makes perfect sense to have the layout as:

User name field
User email address field
User website url field
Comment field
Submit button

But I simply couldn’t figure out why the comment field moves to the top of the sequence by itself. Anyone has the same problem and a solution to this? Appreciate your help. Thank you very much.