To understand the origin of Bekenstein-Hawking entropy, reference proposes a quantitative formulation of black hole complementarity, which states that both the outside fixed position observer and the inside co-moving observer's description of the microscopic state of black holes are equally right and complete. However, hybridising the two will cause errors. In physical black holes, i.e., those forming through gravitational collapse, outside fixed position observers will see matter cores continuously collapsing but never contracting smaller than the horizon size and causing singularity in the central point because the domain of time they use does not cover the range of such events' occurrence. Meanwhile, inside co-moving observers will see periodically oscillating matter cores because the equivalence principle implies that the collapse of matter across the horizon and over-cross the instantaneously singular central point is equally easy and trivial.