It's not so hard: anyone needing to go to HKG at 24 hours notice probably needs to go there urgently, and will be willing to pay that fare. This is particularly true since those with urgent personal needs to go to HKG may well use miles to avoid paying that fare, or may buy from a consolidator rather than CO, so that CO.com is only seeing relatively price-insensitive consumers.. Better to sell 1 seat at $2000 than 2 seats at $800 each, or 5 at $300.

The other issue is the dynamic part of pricing. If I knew that last-minute flights would fall to $100 (marginal cost of fuel and meals, whatever it is) whenever CO didn't sell out, then I wouldn't ever buy in advance at $800 if my travel can be easily rescheduled. CO could well be better off committing not to discounting on empty flights right before departure to avoid this kind of strategic purchasing behavior by price-sensitive consumers. The first reason is likely more important than this one.

What is in common here is that the price is not set to maximize the number of seats sold on today's flight, to the exclusion of all else.