The timings are based around offering the biggest window for connecting flights at both ends...

Typically, QF have their a/c parked at LAX and SFO for 10-12 hours before they make their return trip over the Pacific.
If QF turned their a/c around in LAX and SFO within 3 hours, the flight would get back to Australia late in the evening ruling out any chance of connecting onto any other Australian cities during the same day. All passengers would have to spend the night in a hotel before completing the rest of their journey the following day.
Parking the a/c on the ground in the US until the evening also allows the biggest window for connecting passengers from other areas of US to make a same day connection to Australia.
It does make good sense even though you would think that QF could be doing more with the a/c useage/cycles to maximize revenue...
