If Kirk says "I was born in Iowa", that is now canon (part of the history of the show) from there on in. They can't turn round and say he was born in calcutta.
Yes, but what if Kirk was lying
when he said he was born in Iowa?
Or he simply forgot?
Or he was possessed by an alien entity at the time?
Or he had been ripped in half and that was his evil side that said it?
Or aliens experimented on him as a child and only made him believe
that he was born in Iowa?
Or that it was the Mirror Universe Kirk who said that?
It's not so simple, is it?