If quantum physics theories are correct than there's always some amount of pure randomness in the game, making it impossible to create perfectly deterministic and repeatable system of any significant complexity.
The real answer is that decisions exist in a different frame of reference. Some philosophers are stuck in a model in which decisions were taken by the soul, an inmaterial entity. So if decisions are generated by physical processes, they're an ilusion.
But that's as idiotic as it can be. Our brain is a material system and of course decisions are generated by physical processes inside it. The interesting question is how much of us is malleable and how we can make our decisions to change ourselves and our own decision making process.