I wonder if part of the reason why they schedule the replacement flight earlier (8:30pm) is that, if the replacement goes MX, they have more options for rebooking (including the regularly scheduled 839)?
They normally schedule the replacement flights as early as possible. This half of the year (time difference LAX/SYD of 5 hours) that means leaving around 8pm in order to arrive just after SYD's 6am curfew ends. When daylight savings flips (7 hours difference) it normally means 10+pm departures.
A few weeks back they actually went with a morning departure that got into SYD late evening (nor far before the 11pm curfew). I suspect with the new pilot hours rules that came in a few weeks ago this could be difficult to do without an extra crew. (Minimum 10 hours off/8 hours available for sleep, for a flight that CX'ed close to Midnight, but needs to leave early enough in the day to make SYD curfew - although I'm not sure how the rules work given these flights have 2 full crews)