The general idea is this. Experience is just like any fact in that it is open to an indefinite number of interpretations.
In science, it is a commonplace that theories are underdetermined by data - the data have to be interpreted. C.S. Peirce calls the process of getting from data to theory, "abduction". The same idea comes up again in the work of W.v.O.Quine and philsophers of science like Thomas Kuhn and Paul Feyerabend.
In one kind of language learning theory, it is also a commonplace that children can only make sense of their linguistic environment - what is being said to them and around them - by bringing to it quite a lot of interpretive ability, including the ability to extend their own language skills by heavy use of analogical thinking ("Look, three sheeps!)
In another kind of language learning theory, the linguistic experience of the child is thought of as triggering theories or interpretations which yield working internal grammars. Just as Peirce thought that "abduction" only works because we are in some way attuned to the world we live in, so Chomsky thinks that internal grammar-building is heavily constrained by our pre-tuning, our biological inheritance, even if this is later modified (shaped) in informal and formal educational settings.
An adult person's personality is, at least in part, the product of their childhood experiences. Some people think that it is entirely the product of experience. But even if this is so [I don't think so, but let's assume so], experience has to be interpreted. Unless there is one unique way in which individuals are pre-tuned to interpret experience, then the same experiences can yield different personality results just in virtue of the way they are interpreted. The very same experiences which produce a mean personality in one individual may make another person generous.
This is not a question of "free will". It is a question of what the (growing) individual makes of their experiences. Some - probably most - of this making will be unconscious (triggered, if you like) but at least a small part will be reflected upon.
But even the unconscious part is not entirely predictable: whatever pre-tuning (whatever bioprogram) we start off with, it is not so rigid as to preclude divergent responses to the "same" situations. For this, we may be grateful.