That would change the scheduled departure or arrival times (ISTR they sometimes retime North American departures/arrivals when the clocks change so that the times in and out of the HKG hub remain the same) but I don't see how this would explain the change in the flying time. And anyway, the "watershed" in terms of the dates OP looked at don't coincide with the clock change, which has already happened.

I'd speculate that it has something to do with seasonal changes in wind patterns, but I'd be curious to hear the real answer from someone who knows!