Menu styling different from localhost to live, why?

I’m scratching my head. My header menu has styling on localhost but when i went activate the theme the styling is different. I used firebug to inspect and have found that the tags are different?? Here is an example, on localhost this is the html for the parent tags and first anchor tag

<div id="header-nav" class="menu-header-menu-container"><ul id="menu-header-menu" class="menu"><li id="menu-item-91" class="menu-item menu-item-type-custom menu-item-object-custom menu-item-home menu-item-91 first-child"><a href="https://localhost/" class="menu-image-title-">Home</a></li>

on the live website it is different , the parent tag and anchor tags are different

<div class="menu"><ul><li class="page_item page-item-218"><a href="https://example.co.uk/index.php/about/">About</a></li>

I’ve checked the menus they are identical, why does localhost li attribute say ‘menu-item’ and the live li attribute say ‘page-item’? How can i resolve this?

Here is how i call the menu
wp_nav_menu( array( 'theme_location' => 'header-menu','container_id' => 'header-nav'));

I’ve noticed on the live version the container id hasn’t been changed to ‘header-nav’, why?