Because that is what you have saved in the bookmark, or in the address history.
Later versions of IE and Edge try and be super helpful (and I think this is a dis service) and hide the later part of the address bar. End result, most users do not know where they really are going.
I discovered that when I manually typed the URL into the address field I got taken to the right place, so it's not a browser bookmark issue.
So I went to my RoboForm password manager, from where I always open pages requiring passwords. It displayed lightroomforum URL but when I opened the password "card" it displayed the URL I was being taken to. When I closed the card only the lightroomforum.net part of the URL displayed! So I cleared everything after the backslash. Now it only loads lightroomforum.net and takes me where I expect to end up.