I've noticed this too, and always book via SFO when possible. One thing I have noticed, is that the UD seems to fill up earlier on the SFO flights, maybe because of the high number of UA elites in SFO? Booking over 3 months in advance, I could only get 17A. My preference goes row 15, 16A, 17A, 16A, 17B. Either way SFO is still a better choice for the SYD flights IMHO.