Qantas have worked out a good way of 'enhancing' double SC offers -- narrowing the promotion period (even if there will be another double SC offer later in the year).
It means you have to take a big risk with flights outside the promotion period.
Say I know I need to fly in September. Under the equivalent promotion last year, I would have been able to book that flight now with guaranteed double SC.
Now I have to hold out the hope for another double SC later in the year, but the risk is that work/personal circumstances will force me to book it before that second double SC offer or, if I do hold out, all the good flights will be gone.
Edit: Another disadvantage of the narrowing of the promotion period is shown below.
If you book a return flight that departs in July and returns in September, you only get double SC for the outward flight.