Your assumption is that given a certain set of conditions, only one result is possible. Quantum mechanics suggest that there can be multiple possible results given the same conditions. Some outcomes are of course more probable than others. Also, if we are going to get metaphysical here, it could be argued that a person's "soul" or "mind" could actually be the final determining factor on which outcome actually takes place. Since this soul is metaphysical, it would not be bound by the laws that govern the physical world. A God could then hold this soul accountable for the final decisions it makes.