This is a good question.

The various engines, assuming you mean GDSs, have their origins in the days of text based terminals tied into a giant mainframe over a telephone line. Although the GDS industry has in recent years introduced web based services based on XML command/response strings, even these enhancements still are just reformatted extensions of the old text level command strings.

The result of all this is the information must be asked for piecemeal with responses that are also fragmented. For instance, you can ask for availability of all buckets for a flight or just one bucket, but in one command you can't ask for all business class buckets only. So, for a service provider to allow you to ask for only business class availability might take a series of 4 or 5 queries being issued to the GDS or very intelligent software to filter out just the requested inventory from general request.

So, third-party services try to do the heavy lifting and translate what the you the user want to see, and how you want to see it, into a series of queries to the GDSs and then aggregate the responses into one screen reponse. The problem with this is that it takes a lot of communications time with the GDSs and not all GDSs and airlines speak the same "language". There is also proprietary software that must be written by the service provider to take this data and simplify it in a manner appropriate for their customers.

Hope this helps,

ExpertFlyer Voice