Fares are typically set, as posted, daily.
However, a fare class can fill up. Say N fills up, so M is now the cheapest. (Substitute any letters for discount economy fare classes on your favorite airline.) At that point, the N fare goes away. The M fare hasn't changed, it's exactly what it used to be, but it has gone from the second-least-expensive (which most people don't see) to the least expensive (which shows up in searches by fare). From the viewpoint of a traveler looking for the lowest fare, it's gone up. This can happen at any time.