BA16 usually arrives from Sydney far ahead of schedule. Both BA15/16 usually gets delayed departing fom Changi airport. And should BA12 fail to depart on time, it is more likely to e late since the difference in landing time is smaller than the difference in departure time.

being mega jumbo, more likely to have missing passengers thus need to close the gate later? While those on BA16 could be in a better position to board and depart on time? Just some wild guesses