There is user settings (user metadata) which override the initial meta box order. So it is up to user which meta box he want to display first. User can drag and drop metaboxes and arrange them in any order which he want regardless the order you’ve defined.
You can go to user meta table and find key with name meta-box-order_{post_type} e.g. meta-box-order_page delete the value of this key then check again.
There is nothing wrong with your code.