I am prompted to ask this question following recent experience:

Over a period of three days I was using expertflyer to check sub-class availability for a special fare (mix of O, Q, N, and S class) out of London, to Australia. Using CX's online booking engine in parallel, I was able to confirm these sub-classes were indeed not available for the dates that I wanted. However, higher fares (V, L, M, K and H classes) were available at around double the price. I also called CX London reservation and they story was the same.

In fact, the availability of the lower classes was consistently non-existent over the 4-5 day period I was looking at I eventually gave up trying.
The passengers (I was booking this for family) then called up a local travel agent and it issued them O class tickets at V class prices. I was obviously upset by how stupid they were for buying a non-mileage tix for a mileage price.

My questions:

1) how come the travel agent was able to book O class when CX's own online booking and phone reservation system couldn't? [Pls note: CX does offer this "O" class fare through both its reservation and website and in fact I did find availability on some undesirable dates]

2) I can't find any references but presumably CX doesn't guarantee cheapest fares online?