Any flight more than about 60 days out with Delta is in actuality just a “placeholder” and may/may not actually end up operating. As the dates get closer the airline finalizes schedules based on demand and aircraft/crew resources. Even inside the 60 day mark minor changes in time or aircraft type might occur. The best strategy is to book a flight that works for you and monitor the reservation for schedule changes every Saturday. When a change occurs, you can make a free change to another acceptable delta flight provided it meets certain criteria (see the master schedule change thread on here for more info.) Also, another popular technique is to book whichever flight is the least expensive when booking far out in the hopes of being able to switch to a more desirable flight for free once a schedule change occurs. Of course this is not guaranteed, but can definitely pay off. And off topic, but I don’t recommend basic economy as it can make this practice a little more difficult. Always buy main or above.

Lastly, this practice of making schedules essentially placeholders seems to be more common with US carriers. Yes, it can and does happen with other carriers as well, but with airlines like Delta schedule changes are literally a weekly thing.