that is the nature of quantum physics. Even if the initial conditions would be exactly the same, every collision would have a certain number of possible outcomes (not just one, many!), and there are probabilities (chances) that can be calculated by the physics theory we call the Standard Model, which makes predictions on how often each kind of collision happens. The interesting collisions are really really really rare. But even 'normal' proton-proton collisions are all slightly different.