It's because of the forum descriptions. The very last one one the second category is longer than all the rest so the width of that column is going to be bigger.
You can change this by replacing
Forum Bit Templates >
forumbit_depth2_forum with this code:
<tr>
<td class="{$bgcolor}" align="center" valign="top" width="1"><img src="{$theme['imgdir']}/{$lightbulb['folder']}.gif" alt="{$lightbulb['altonoff']}" title="{$lightbulb['altonoff']}" class="ajax_mark_read" id="mark_read_{$forum['fid']}" /></td>
<td class="{$bgcolor}" valign="top">
<strong><a href="{$forum_url}">{$forum['name']}</a></strong>{$forum_viewers_text}<div class="smalltext"><span class="forum-desc">{$forum['description']}</span>{$modlist}{$subforums}</div>
</td>
<td class="{$bgcolor}" valign="top" align="center" style="white-space: nowrap">{$threads}{$unapproved['unapproved_threads']}</td>
<td class="{$bgcolor}" valign="top" align="center" style="white-space: nowrap">{$posts}{$unapproved['unapproved_posts']}</td>
<td class="{$bgcolor}" valign="top" align="right" style="white-space: nowrap">{$lastpost}</td>
</tr>
And then add this anywhere into your CSS:
.forum-desc {
width: [your specified width here];
}
Obviously replacing the phrase in braces to your own width.