Why isnt themed Tabs (add) show the default type?

Why isnt themed Tabs (add) show the default type?


I have tabs set up using the look and feel of the roll your own
themes.
When adding a new tab using
$('#tabs2 > ul').tabs('add', 'source of AJAX response', 'NAME');
the new tab shows and works - but the look and feel is that of a link,
not the themed class i.e. <li class='ui-tabs-nav-item'>.
Am I missing something?