The reaction of one man could be forecast by no known mathematics; the reaction of a billion is something else again.
This principle of Seldon’s psychohistory is part of what makes the concept compelling (compelling as a tool in his book). He doesn’t just adopt determinism, he respects free will. He is developing a system where, somehow, free will is acknowledged but all the geeky sci-fi mathematics of determinism can still be utilized. It also allows for suspense/conflict which is naturally there between the two contradictory ideas (what one will win?).
