Why does flight availability differ between travel web-sites?

The other day I was booking flights for a winter trip. A friend who is an expert at all things travel found me a good fare with connections and layovers to match by using ITAsoftware.com. ITAsoftware doesn't actually do the booking so we headed over to Cathay Pacific (the carrier with the best flights for my needs) to make the reservation. Unfortunately, Cathay Pacific showed the same flights at a much higher price. With rising concern, we tried Orbitz. Orbitz showed these flights at the good (ITAsoftware) price and it looked liked we could book the flights. Only, when we tried to buy the tickets, Orbitz claimed that "availability had changed" and we could not complete the transaction. I repeated the process 12 hours later with similar results. Why does flight availability differ between these web-sites?

Presumably all these web-sites access the same back-end databases. Thus, in principle, they should show the same flights at the same price (minus any booking fees). Another friend offered that the difference is likely due to travel agents and potential travelers booking flights with 24/48 hour holds on tickets. The discrepancy between the web-sites thus boils down to the differing interpretation of the back-end database. In my case, Cathay Pacific's interpretation likely is "We try our hardest to honor holds". Orbitz's take looks to be "We show you what is possible, even though you may have to wait a while to get it". And, ITAsoftware seems to say "These tickets are available because technically nobody has bought them yet".

These differing interpretations are frustrating partly because they are not spelled out in a noticeable way on the respective web-sites. The best that a traveler can do then is wait a couple days (when the holds presumably expire) and try again. Perhaps airlines should enforce a consistent interpretation of their databases - without consistency, the airlines are (probably) inadvertently pushing travelers with somewhat involved itineraries, or equivalently limited patience, towards travel agents.

UPDATED 8/7/2010
For a more recent article on "fake ticket prices" go here.

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