Originally Posted by Keter

There are several other reasons I can think of that can influence availability and some of those are nearly impossible to emulate in Expertflyer...

The reasons include:

1) Point of sale (PoS) - availability in lower classes may significantly differ for sales via say US or UK (BA is good in that).
2) Further advancement - availability is depending on the point of origin or even a mix of parameters from your itinerary (including PoS).

To illustrate #2 say you are looking into the availability of NYC-PAR. The resulting availability will be different if your itinerary is
a) NYC-PAR-NYC and
b) PAR-NYC-PAR.
Moreover, the result can be different if you want to make changes to an existing reservaiton or making a new booking (this is how QR now requires to make changes to their bookings).

Another example is booking a trip with stopover on EK. EK requires that agents book stopover itineraries using the availability for a through itinerary on your departure date (if this is done manually of course).

I would say it becomes really difficult for a passenger to know what the 'applicable' availability is for his/ her case...