It shows not specified since the user has selected the default style as his/her style in the user options.
Now... There is a way to remove it very easily and display Default Style instead... Just a small error. You see, in the postbit, the if conditional it says: == '$vbphrase[not_specified]' while in the file you edited in: '(' . $vbphrase['not_specified'] . ')' Duh! They don't match

.
What I suggest is....
Change this (in includes/functions_showthread.php):
PHP Code:
// check what style the user is viewing
if (!$post['styleid'])
{
$post['style'] = '(' . $vbphrase['not_specified'] . ')';
}
Into this:
PHP Code:
// check what style the user is viewing
if (!$post['styleid'])
{
$post['style'] = 'default';
}
Change (in your postbit or postbit_legacy):
HTML Code:
<!-- user forum style -->
<div>$vbphrase[style]: <if condition="$post[style] == '$vbphrase[not_specified]'"><a href="$vboptions[forumhome].php?styleid=$vboptions[styleid]">Default Style</a><else />$post[style]</if></div>
<!-- / user forum style -->
Into:
HTML Code:
<!-- user forum style -->
<div>$vbphrase[style]: <if condition="$post[style] == 'default'"><a href="$vboptions[forumhome].php?styleid=$vboptions[styleid]">Default Style</a><else />$post[style]</if></div>
<!-- / user forum style -->
Now, with this you do not have to add 'not specified' to the phrases.

I mean, after all - the Default Style will be displayed, not the phrase or word that was put in the PHP as... ah, I'll shut up

.