It is possible for a console operator to sell tickets from any point to another. I do this on occasion to buy tickets to "extend" my periodical zone ticket to a particular station.You don't even have to ask. Just push a button on the ticket machine. I had the impression that a station can only sell a ticket to/from that station. At least that's what I was told once. Are you sure that wasn't the price from St James?
I just walked downstairs to Martin Place station, and asked for a single from Mascot to Domestic Airport. The price was $15.90. I could see the operator's screen to verify the price was from Mascot.
I did not know that Cityrail varied fares depending upon the point of sale. Something new learnt from AFF today!