You may lack an understanding of the practice known as married segment availability, or journey control. For any ticket, be it award or purchased, availability aaa-bbb-ccc does not guarantee availability aaa-bbb or bbb-ccc.

Apparently your aaa-bbb-ccc itinerary is something DL is willing to price at 60K miles, but aaa-bbb is 75K. Regrets, but (again, both purchased and award) this happens every day with U.S. airlines. They find lots of reasons to do this such as 'being able to complete with airlines flying non-stop aaa-ccc,' but the reasons have little appeal to travelers.