It seems to me like the DADA class syllabus is not set in Hogwarts. In book 2, Lockhart set all his books as the year's syllabus for the class. In book 5, Umbridge set her own syllabus - sort of pre-Hogwarts baby books for fifth, sixth, and seventh year at Hogwarts. 

Although the OWL examiners had their expectations from the 5th year students, and students were judged on those levels, it seems like what happens inside classes could be as irrelevant to the syllabus and nobody would bother. 

What was even more strange that in book 2, when Harry and Weasley's received their Hogwarts letter, Fred/George peered down Harry's list of books and said that ***You are also told to purchase all Lockhart's books***. This proves that not only the second year students, but the students in the higher classes were also given a similar syllabus, including students of OWL and NEWT classes. How come?