FWIW there's also two relatively close daily flights SFO-PVG (857 departs around 1:20-1:40pm; 891 departs around 2:30-2:50pm) while on the return leg they are about 12 hours apart (890 departs 12:15-12:45am; 858 departs 1:15-1:45pm).
My guess is that is has to do with connection times and airplane utilization/servicing needs. But that's just a guess.