Panels Tabs Doesn't follow 'Artisteer generated theme' style

Here's the pic.

At the top: normal tabs.
At the bottom: tabs with weird style.

This is the corresponding faulty code:

<li class="upcoming ui-state-default ui-corner-top first">
<a href="#upcoming">
<span class="tab">Upcoming</span>
</a>
</li>

This is the code in Garland, that doesn't work with Artisteer:

<li class="upcoming ui-state-default ui-corner-top ui-tabs-selected ui-state-active active first">
<a href="#upcoming">
<span class="tab">Upcoming</span>
</a>
</li>

This is the working one for the top tabs:

<li>
<span class="art-button-wrapper">
<span class="art-button-l"></span>
<span class="art-button-r"></span>
<a class="art-button" href="/node/9/news">News</a>
</span>
</li>

Normal tabs in Garland (which also work with Artisteer):

<li class="upcoming ui-state-default ui-corner-top ui-tabs-selected ui-state-active active first">
<a href="#upcoming">
<span class="tab">Upcoming</span>
</a>
</li>

To me they seem to be very similar.

Does the problem come from it uses li class insted of span class?

Any idea how to solve this is highly appreciated.

http://drupal.org/node/1026866