Well first off we are seeing more and more that there are quantum effects in the brain, and we don't know what they all do or what role if any they play in conciousness.. We just know quantum shit is random and hard to predict while only really affecting things on the smallest possible levels. Second some elements are hard to predict because two things are equally probable.

Like if I go to get Ice Cream and I really like strawberry, but they're out and I can only choose Vanilla or Chocolate, then it's a 50/50 and the only thing that decides which I choose is my own decision.

Some things simply cannot be measured.

How do you record a dream? How do you measure someone's luck?