Monolayer materials are strongly affected by their potential fluctuations, which can be induced by an intrinsic corrugation or the surface roughness of the substrate. We compare the effective exciton-exciton annihilation (EEA) rate constants of monolayer WS2 on substrates with different surface topographies. We show that the WS2 monolayers on the substrates with atomically flat terraces have small effective EEA rate constants that deviate from the overall tendency and exhibit multiple exciton decay components, which cannot be accounted for by the conventional EEA model. To obtain a correct description, it is important to use a quantized EEA model. The intrinsic EEA rate constants for the flat-terrace substrates determined by this model are comparable to that of hBN-encapsulated monolayer WS2.