That's how it is scheduled. CX840 turns around as CX889, CX888 turns around as CX841. If you look at the schedule, CX840 would have to sit at JFK overnight in order to fly back as CX841 while CX888 would have to sit at JFK all day in order to fly back as CX889.

It is not unique. For example, some NRT flights are like this. HKG-TPE-NRT-HKG (CX450/CX505) and HKG-NRT-TPE-HKG (CX504/CX451). A delay of CX450 TPE-NRT will cause a delay of CX505 NRT-HKG.