When an airfare sale is not quite a sale

How do airlines decide what fares qualify as “sales,” and why do they advertise certain fares, but not other, much lower ones? Why is United Airlines promoting a “sale” between Washington and Boston for $109 each way, when there are currently six published lower fares in that market, beginning with $49 each way?

For the most part, I don’t bother to figure out why airlines do certain things anymore. I just gather all the information I need about what they do and try to work with it — or around it. Years of watching fares have taught me not to fall for those “sales,” because in many cases, I can find a much lower price to the same destination, on the same dates and on the same carrier…