I'm looking for a flight from Cape Town (CPT) to Atlanta (ATL), and I see some I like on both Matrix Airfare Search and Orbitz (thanks to answers on my other question:

But when I try to book the flight on Orbitz, I'm told the flight is no longer available:

I would assume this means the last flight at that price was just recently booked, and I'm out of luck.
Immediately after selecting a flight, and being shown the yellow banner above, if I do another identical search, the missing flight is removed from the list.
If I come back an hour later (as confirmed just now, for the flight above), the phantom flight has returned. I have experienced this behavior consistently now over the last week or more.
It's not just a problem with a single airline, either. When I'm returned to the list of results at Orbitz, the top 4-6 cheapest flights do the same thing, from a variety of airlines. And it happens on all dates (I've tried Dec 2013, Jan 2014 and even as far out as March 2014).
What gives? Where are these phantom flights?