We perform spectral analysis based on suitable theoretical correlations between V-wave visibility and D-particle distinguishability to verify wave-particle duality in the asymmetrical double-slit experiments. The typical interference spectra obtained in a recent experiment ensure the single-photon approximation in the Fraunhofer far-field condition. The results show that our theoretical model following the complementarity principle is not be consistent with the experimental data at the first main minima of the diffraction distribution where unexpected interference was observed