Hi Peter,
This is not a bug, but the only way Questionnaire can deal with your answers editing. I would certainly strongly discourage editing questions once users have started to answer.
Anyway, the response analysis goes by choice positions, i.e. if a user selected "Blue" (position 3) in the original version of your questionnaire, and you edit to replace choice "Blue" with "Yellow" in the same position 3, then of course that user's response does not change (it's still position 3) but it now displays the new "color" you have painted over Blue, i.e. Yellow.
Joseph