You've stumbled onto a glitch in the award calendar at delta.com. There have been many posts regarding this, but long story short the website is dismal as far as award availability and data reliability.
As I see it, you have 3 choices:
1) Take what the website offers,
2) Call the website support desk in Upper Sri Lanka and try to decipher their explanations. or
3) Call SMS and be prepared to cough up some coin.