[Plugin] Getting an extra sub menu item in my plugin menu

I’m having a weird problem with a menu I’m adding to the administration interface.

I keep getting an unwanted sub menu item.
I have the follwoing code:

add_action('admin_menu', 'sl_menu');

function sl_menu()
{
  if (function_exists('add_menu_page')) {
		add_menu_page('Menu title','Menu title', 'manage_options', 'my-pluging-folder/my_plugin.php', '', '');
	}
	if (function_exists('add_submenu_page')) {
		add_submenu_page('my-pluging-folder/my_plugin.php', 'Sub menu title', 'Sub menu title', 'manage_options', 'my-pluging-folder/some_file.php');
  }
}

The result is:
Menu Tile
– Menu Title
– Sub menu title

As you can see, the main page / menu tile is also shown as a sub menu item.

Why is that?

If I take “the same” code from Lester Chans plugin ‘wp-polls’, I only get the main menu item + one sub menu item:

### Function: Poll Administration Menu
add_action('admin_menu', 'poll_menu');
function poll_menu() {
	if (function_exists('add_menu_page')) {
		add_menu_page(__('Polls', 'wp-polls'), __('Polls', 'wp-polls'), 'manage_polls', 'wp-polls/polls-manager.php', '', plugins_url('wp-polls/images/poll.png'));
	}
	if (function_exists('add_submenu_page')) {
		add_submenu_page('wp-polls/polls-manager.php', __('Manage Polls', 'wp-polls'), __('Manage Polls', 'wp-polls'), 'manage_polls', 'wp-polls/polls-manager.php');
	}
}

Now, why does this work?