Actually it's not a website quirk and has nothing to do with the website.
I can give an example from AA (who has dummy flight numbers galore in its system) as I'm more familiar with it and have used Sabre when I worked there....

The way Sabre is set up, it's easiest to store a lot of documents under a flight number. Stations will load a dummy flight number between imaginary points and then store documents under that flight's information, like memos for agents, announcements, etc. Some stations set it up so each agent has a mock PNR on that flight so they can store information for their own use. These dummy flights can serve a variety of other purposes, like the ones AA uses to generate gate passes (like St. Louis to Anytown for an Admirals Club gate pass I recently saw). They also can serve as test flights to run through the system for checks. Because of the way the computer system is set up, it acts like it is a real flight (just can't really book it for a passenger, haha) and thus will show up in flight information queries. It's 40 year old technology at its best! I'd assume the uses at US are very similar.