When performing non-reacting mixing enhancement experimental studies in supersonic flow, helium is often used as an injectant which serves as a simulant gas for hydrogen fuel. The resulting distribution of helium mole fraction in the binary mixture of air and helium can be quantified using filtered Rayleigh scattering (FRS). The FRS technique requires two independent experiments in supersonic flow, one with helium injection and the other with air injection. The helium mole fraction is retrieved under the key assumptions that the number density profiles and the extent of the Doppler shift that is associated with each of the two independent experiments at the measuring plane are identically the same. This work is centered on the analysis of the impact of a departure from the aforementioned assumptions with the aid of a reduced-order model developed for a canonical rectangular jet in supersonic flow. The results, derived from the implementation of the model, are used to identify key driving phenomena that concurrently contribute to violate the key assumptions and are compared and discussed in the light of available experimental data. Additionally, the analysis suggests that the newly developed model can be used in the design of FRS experiments by minimizing the extent of mismatch in the number density profile and thus reducing the systematic bias error associated with the mixture's composition measurements.