It was quite awhile ago, so I can't recall the fare details from that specific trip, but I re-created it using WAS as the destination, and this is what I found:

NOTE - THE CITY PAIRS BELOW ARE CONSIDERED THE SAME POINT-
EFD-HOU EFD-IAH HOU-IAH IAD-DCA MDW-ORD LGA-JFK

The routing for the lowest fare was an X class going to IAD - X class is not published from EWR to DCA, only to IAD. The lowest class published to DCA is U, which prices higher.

A fare class is not published to an airport I want to standby to, yet the fare class I am booked in states the ticketed airport and my requested airport are actually the same point.

So...which over-rides which? The fare rule or the class of service?