It isn't quite the case that all assumption questions are necessary premise questions. Rather it is that all strengthen with necessary premise questions are required/necessary assumption questions and that all strengthen with sufficient premise questions are sufficient assumption questions.
To differentiate between strengthen with necessary premise and strengthen with sufficient premise, note the language used in the question stem:
Strengthen with necessary premise - "assumption/presupposition" combined with one of the following "must/required/depends/necessary"
Strengthen with sufficient premise - use of the sufficient condition indicator "if", and the one of the following "allows the conclusion to be properly drawn/inferred, justifies the conclusion"
Different names for the question types are arbitrary. The techniques that apply to each, however, are not. So, hopefully this has cleared up the confusion about identifying the question type with regard to strengthen with necessary and sufficient premise.
Please let me know if you have any other questions!