Actually you are right. I am in fact talking avout the second case you are describing. Im not sure if it makes sense every time. Furthermore, consider this case:
John sends a message to Terry. He puts me and Wendy in the cc section. When I reply all, John and Terry goes into the To section and Wendy to cc section. This logic does not really makes sense. It does make sense in only few cases I would say. Normally when Im in cc and I want to reply all, I want to do that for the sender, not his original recipient (my message is not to Johns recipient, Terry).
Now this case is most troublesome for me:
John sends a message to me, Terry and Wendy (To section). When I answer all, I would expect that only John will be in the To section while Terry and wendy will go to the CC section. Instead, I answer to all 3 of them (To section).