In Bell-test experiments two kinds of probabilities appear: calculated probabilities and measured probabilities. The calculated probabilities (Bell's) cannot be detected by the detectors, whereas the measured probabilities (QM) are being detected by the detectors indirectly. The two kinds of probabilities are very different but both can be accounted for in a local-real way. They both can be made visible as projections of one and the same vector space in different directions. The differences between the kinds of probabilities are caused by the fact that detectors and observers perceive things like: vectors, vector spaces, particles and pairs of particles, differently. Observers perceive calculated probabilities (Bell) and detectors detect measured probabilities (QM) but only indirectly. In the paper both kinds of probabilities are being defined exactly. The exact definition of the probabilities makes clear why they appear as they do.