Freysa tells Joe, shortly after revealing that he is not the born child:
We all wish it was us. That's why we believe.
What is this sentence supposed to mean?
The first, most simple interpretation is that each of the rebels believes they're the child, presumably as a source of strength to fight on. Obviously it can be a lie to oneself at most, as Freysa knows the child is not among them (I don't remember if it's implied she actually knows its exact identity, but overall we can assume she does know the child is not there). It's also a possibility that, to uphold hope, she told no one of it (better to lie than to lose hope), although the crowd stayed perfectly calm during the revelation, so we can assume it's not true.
During the movie I thought that Ana (the true child), aside of multiple faked memories, every now and then grants a replicant a memory of her own, one that suggests they are the child born. However, at least in Joe's case, the memory required pretty much an improbable sequence of events to uncover its true meaning - without it, the memory appears as yet another meaningless fake, which kind of defeats Ana's purpose and the correctness of this thesis. We might say that Joe's memory was far harder to understand than the rest, although I find a rather implausible explanation.
So, what's the deal here?