I have some confusion about multiple scattering. We always say that the problem of single scattering is always deterministic in nature.But while modeling the problem of multiple scattering, we take that the problem is stochastic in nature. I don't understand why. Why multiple scattering/collision is considered as stochastic? We can exactly say what will occur when a single electron or a photon collides with an atom/molecule/aerosol. We can use the cross section values for various possible processes like ionization, dissociation etc to find the probability of occurrence of different processes. But while studying the case where millions of electrons or photons are coming and colliding with molecules or atoms, why we say that processes that occur will be random ?