Something I'm wondering is related to the rumour and also the video footage we saw of Ferrari during testing where the car would seem to dip excessively under heavy braking - there were rumours of them losing drag on the straights and then running closer to the ground in the corners for more grip. I don't know if you remember Vettel going off at turn 3 (I think it). At the time I wondered whether something had gone wrong with this dipping under heavy braking - maybe the car had dipped too much. Maybe they were worried about this, especially at a bumpier circuit like Albert Park and so they had to undo this development. Mark Hughes did say a lot of their time loss was under braking and traction out of slower corners.
The other theory is their PU running in a conservative mode because of the reliability issues they had relating to that in testing. Certainly the back of their bodywork was opened up a lot more since testing.