Changing the answers to a question, invalidates old results.

1. create a question: 'question 1'
2. create 3 answers 'a','b','c' (answer 'b' is correct)
3. Run the quiz & submit one or more answers to this question
4. edit the question, so the answers now read 'a','c','b' (and still mark 'b' as correct)
5. review the responses to the question - the students who previously, correctly, answered 'b' (option 2) are now marked as having answered 'c' (the new option 2).

I am not sure there is a sensible solution to this, other than the option to remove old submissions when editing questions.