HKs are often misunderstood (the symbol, not the people). From non-rev travel FAQs:

2. I often see a large number of "HK" and revenue standby passengers appear on the airport standby list before a flight, which, in many cases, showed plenty of available seats the day or even hours before. Who are these passengers and where do they come from?
HK passengers are customers who have confirmed reservations but were not assigned a seat when they purchased their ticket or checked in – they are placed on the waitlist and assigned seats at the gate. HK passengers do hold a confirmed reservation on the flight, and the seats they will occupy are already accounted for in the available (AV) seat total in TravelNet.
There are several reasons why you may see HK passengers listed on your flight. For example:
• They may have purchased a Basic Economy fare which does not allow advance seat selection
• They elected not to purchase a Comfort + or other premium seat, which may have been the only seating options available at the time they purchased their ticket.
Revenue standby passengers are passengers who are confirmed on a specific flight but are choosing to standby for a different flight. They are represented on the airport standby list by standby codes like B1, SDSB and SDSC. Unlike HK's, the seats these standby passengers will occupy are not accounted for in the available (AV) seat total in TravelNet.
There are many reasons why revenue standby passengers may appear on the airport standby list close to the flight's departure time. For example, they may have been rerouted on to the flight by Delta's automatic rebooking tool, VIPER, if their earlier flight was delayed or cancelled. Or they may have also taken advantage of the flexibility and convenience of Delta's Same Day Travel/Same Day Confirmed program, which allows them to change their flights or standby for another flight without paying a fee.
3. Oftentimes a single flight will show many seats available on one day, but few to no seats available on the next. What causes our passenger bookings to change so often? Does Delta offload seats at online booking sites?
Delta does not offload inventory to online discount booking sites. The following are some common reasons why passenger seat bookings may change on a given flight:
• When an irregular operation occurs due to weather or other circumstances, revenue customers are rebooked on whatever flights are available to get them to their final destination.
• The aircraft type scheduled to operate the flight may be switched for a number of operational reasons, and the new aircraft type may have less (or more) passenger capacity than the original one.
• The flight may have been given passenger weight restrictions for a number of aircraft performance reasons, or may have been payload optimized to ensure that important revenue cargo can be accommodated.
• Revenue customers may have changed their travel plans to take another flight.