Yes it is down to the environment but right now we don't know what these environmental exposures are. I've always considered "chance" and "luck" in science to simply be a placeholder for the unknown.
- we know with certainty that it follows a <xy> distribution and there is no way we could know more
- we don't know enough so it appears random to us.
If I were forced to bet on one of these, I'd say it's always the latter except in the case of Heisenberg uncertainty.