Conditional Formatting and Template Tags

Hi guys
I have this code:

<div id="masthead_image">

        <ul>
        <li <?php if(is_front_page()) { ?>class="current_page_item"<?php } ?>><a href="<?php echo get_option('home'); ?>/">Home</a></li>
		<?php wp_list_pages('sort_column=menu_order&title_li=' ); ?>
		<?php wp_list_categories('orderby=name&title_li='); ?>
      </ul>

      </div>

and the CSS:

div#masthead ul li a {
color:#35044c;
text-decoration:none;
display:block;
font-size:1.3em;
font-weight:700;
text-transform:uppercase;
height: 40px;
width:150px;
}

div#masthead ul li a:hover,div#masthead ul li.current_page_item {
background:#00ff00;
}

I understand why the home button is shown as active, as there is the query in the code. But why does the page get shown as active, but the categories don’t?
Live version: http://www.forecourt.org.uk

Thanks in advance 🙂