Wordpress - Get page IDs from nav items

Menu items are stored in the posts table with a post_type of nav_menu_item. So, what you are returning is the ID of the menu item itself, not what it points to.

The page/post ID that the menu item refers to is stored in the postmeta table, with a post_id that matches the menu item ID and meta_key = _menu_item_object_id. The easiest way to get the meta_value (ie the page being pointed to by the menu item) is to use something like this:

$numbers[] = get_post_meta( $menu_item->ID, '_menu_item_object_id', true );