It is evident that while small contributions to the mismatch occur due to the photocycloaddition reaction kinetics at much shorter wavelengths, it does not explain the much larger shift in reactivity when compared to the absorbance spectrum at wavelengths above 400 nm. Simply, an additional and dominant mechanism must be causing a wavelength-dependent quantum yield for the photocycloaddition that is not operational in the photoisomerisation. We below present a theory and experimental data in its support to explain the mismatch.