The highlight indicates the current menu item. However, as you have multiple menu items pointing to homepage sections (technically also the homepage), WordPress treats them all as being the current menu items.
You can remove the highlight with custom CSS:
body #access ul > li.current_page_item,
body #access ul > li.current-menu-item,
body #access ul > li.current_page_ancestor,
body #access ul > li.current-menu-ancestor {
background-color: inherit;
}