I too understand the meaning but find that the given translation ("Am I not writing?") is but one of several variations that could be used because the literal word-for-word translation of this sentence doesn't work with colloquial English. Could also be something like the dated English variation, "Write I not?"
I would use it, if someone sees me writing and still asks me to do something less important. Especially, if before, they were telling me to get the writing done. Next, if they still don't get the hint that I am busy, I would say, "You can see that I am busy. That will have to wait."
This situation could not use "I am not writing?" which is only when someone tells you to write when you have already started. So actually, "I am not writing?" is the sarcastic one or said in disbelief implying that you are telling me that I am not doing what I am actually doing.
"Am I not writing?"is more dramatic and it implies, "You can see that I am doing something more important." The drama comes from the fact that someone is bothering you while you are trying to get something done, because they want you to do something else that you think is less important.
The computer is making up sentences from the vocabulary that we have been given so far and it is tasked with giving both negative and positive questions using every pronoun, so give Duolingo a break.
Nice one, ameka.sam! :D
And Rapoona: yes, technically speaking, any string of words that ends with a question mark is a question, but that way of constructing questions might be a bit too easy, and I think Duolingo wants to make sure that all learners understand that in English (and in Dutch) questions Subject and Verb should be inverted.
Someone else pointed out that "Am I not writing?" should really use the Dutch form "Ben ik niet aan het schrijven?" and I agree even though sometimes we use the progressive form when Dutch might not, this is one of those questions in which we are emphasizing that it is happening right now. Unless of course, the meaning is used that was given by another person above who was told he wasn't going to write the script any more.