Discounts on multi-airline fares such as DGLOB34 are often based on the mileage proportion allocated to each of the airlines. Our corporate discount is higher for QF component than it is for AA, BA, CX etc. The discount is a percentage reduction in the fare component for each of the carriers. The total fare price is divided by the proportion of miles on each carrier, and then each carrier's percentage discount is applied to their portion of the fare.
This means that we get a bigger discount if there is more miles flown on QF flight numbers (including codeshares). In fact, I once worked out that my total fare would be about $100 lower if I changed the final leg of my DONE4 from MEL-BNE to MEL-PER-SYD-ADL-BNE - not to mention the extra SC's. But alas I was more concerned with getting home than wasting my time in a severely jet-lagged state to save a hundred bucks (even though it was my money that was being saved).
The TA's like FC may have a very different way of applying discounts, but it may be based on a similar carrier proportion basis - ie they may get more commission for QF component vs AA, BA etc. So it may be worth asking if there is anything you can do with your itinerary that may affect the discount they will offer. Your TA may not be willing to tell you how they work it out and may not be willing to invest the time into calculating the options. But it may be worth asking.