I am sure there must be a simple explanation

I just checked on flight times from LGW to HKG in November. There is only one flight a day, CX344 which leaves at 11:45 every day.

However, on a Monday and a Friday it arrives at 06:55 - duration 11hrs 20mins. On a Sunday & a Wednesday the arrival time is 7.05 am - 10 minutes later. But on a Tuesday, Thursday & Saturday it does not arrive until 07:55, increasing the flight time to 12 hours 20mins.

The same aircraft, A350, is used on all flights.

I am just curious as to why it is scheduled to take an hour longer on some days