This is deliberate. They don't 'forget' one writer. This system is used is to stop candidates guessing the pattern of answers.
There are four writers and four questions. This task would be far too easy if every set of answers were B,C,A,D or C,A,B,D for example, wouldn't it? That would give us a very limited range of possibilities. If everyone knew that each writer was the answer to one question, candidates would do the first questions and then guess the rest. You'd put D, A and B for the answers to 1 2 and 3, and then you'd guess that the answer to 4 must be C.
Exam-writers don't want this happening, so one writer is nearly always the answer to two of the questions. It makes sense, doesn't it?