Bottom on post navigation

When I press “Read More” and the entire post shows, at the bottom there is an arrow pointing left and one pointing right. I assume they will point to the post before and after the current post. But then I added a second menu tab/item (‘reviews’) and attached a page to that menu item, then attached a post to that page…the arrows at the bottom of the post for the first menu item now pointed to the post in the second menu items (‘reviews’). I guess I created a second menu item and assigned things to it wrong (I’m new to WP). What did I do wrong.

These buttons at the top point to posts from one to another based on date you have published them and they don’t take into consideration where you have added them in menu or in which category this post is published. There is no relationship between anything else than just “date published”.