In principle, there are three possible explanations for why this happens:He says that option 3 is ruled out by the Bell test experiments.
1. Properties of these quantum particles are not real, as we understand "real" numbers. (See, for example, the "i" in the Schrodinger equation.)
2. All properties of quantum particles are real, but there is non-local phenomena, sometimes colloquially called faster-than-light transmission of information.
3. Or, perhaps everything is real and nothing gets transmitted faster than light. Called local realism, there must be some sort of "hidden variable" that -- although we do not observe it -- determines what the final states are.
Einstein's mistake was to say that quantum particles are real, I guess, and to deny that God plays dice.
The double slit experiment is the proof that electrons wave wave properties. I don't think that it says anything about reality or determinism, unless you don't believe that waves are real.