There’s a difference between there being space available in a “fare bucket” and there being an underlying “fare” for that bucket. For someone to sell you a ticket, you need both.

A “fare” is basically an offer from the airline saying “We’ll sell you a ticket right now for this much money if there is space in the S bucket for any flight between DCA and ORD on this date that meets some conditions.” You can then use the search you performed, the Flight Availability search, to see if there is actually S space available for you to take them up on that offer.

However, the availability of space on a plane and the availability of an offer to sell you a seat for a price if that space is available are two completely separate things. You can have either one without the other.

In this case, you did a flight availability search. It came back and said there is space in the S bucket. However, that is only half the question. To actually be sold a ticket, you also need a fare that uses that space. To look up what fares are available, you can use ExpertFlyer’s fare search. In this case, there doesn’t look to be a fare for this route, period, that uses S space. The lowest fare available uses L space. So, that’s what AA.com offers.