Well, I mean, I have in the past written stories on things I did not specifically offer because I wasn't in the frame of mind to remember them / they did not fit into the limited humber of offers when I was signing up. So I matched on something else, and inspiration sparked for entirely different prompts. It's definitely not like you're limited to what you offered - the trick rather seems to be to try and form your offers in such a way as to present the range of what you are comfortable writing. ;-)