It's not a text value, it's the title of the most recent blog post.
If you would like to stop the page/latest post title from being added to the site title, just edit "vertex/cms_core_functions.php" and comment out (add "//" to the front of) or remove
else{$header_html .= the_title('',' on ',false);}
from the "s5_head_call" function.