Yes, this is different from being accepted for publication (instead, it's closer to not rejected). It means they might publish it soon, they might publish it at some point further in the future, and they might not publish it at all. I have no idea what the statistics are, and they may vary depending on the current editor in chief.
Here's some background:
The Mathematical Association of America publishes several widely read, high-quality journals that focus on exposition rather than original research (most prominently the American Mathematical Monthly). There's a lot of competition to publish in these journals, which means they reject many submissions, including almost all submissions to the Monthly.
The Monthly has a tradition of using short, interesting submissions as filler (and I think other MAA journals do as well, but I'm not certain). This tradition arose in the print era, when it was more important to ensure that the number and layout of the pages came out right, and it has continued since readers enjoy the filler and there aren't many good places to publish pieces like this. The filler articles are generally not considered particularly important, and they are chosen to fill the available space, to complement the main articles, or just because they caught the editor's eye.
So why would someone allow their submission to be used as filler? One reason is that there may not be other good opportunities for publishing it, but I think the general reason is that the author doesn't consider it a big deal. The typical filler article is short and entertaining, but not a major part of the author's career. If your submission really matters to you, either because you would feel insulted to see it used as filler or because your career would benefit from having it published soon, then the filler archives are probably not a good place for it. (You might get lucky and see it published immediately, but probably not.)