How is "our" Olivia the correct Olivia?

I’m about half way through the fourth season of Fringe, and I have been noticing a bit of contradiction. I would assume J J was leading up to a plot twist, but even the characters appear oblivious to something that to me looks pretty obvious.

There is a running theme that Peter and Olivia are meant to be together. In fact, when Peter enters September’s mind, September goes as far as to tell him that he had a son with the "alt" Olivia; and that the "alt" Olivia was the wrong Olivia.

But how could this be the case? Peter was brought to our universe only as a direct result of September distracting Walternate when he was working on curing Peter’s illness.

September has made it very clear that this was also not suppose to happen; he has been trying to "fix the timeline" ever since.

But if Peter was never suppose to be brought to our universe, he would have stayed in his universe with the alternate Olivia.

How is our Olivia the right Olivia, when Peter was originally suppose to be in his own universe with alt Olivia?