We demonstrate the first comprehensive explanation for the mismatch observed between absorption spectra and wavelength-resolved photochemical reactivity. While a complex interplay of many effects such as chromophore purity, concentration of sample, and photostationary states, and more (see Section S10 of the Supporting Information) may contribute to this difference, we submit that the rededge site-selectivity dominates, at least in the currently explored example of a [2+2]photocycloaddition. We note that the REE is particularly effective for bimolecular reactions, where excited-state lifetimes are most important. Each system thus depends strongly on the chromophore and surrounding environment, with more significant differences in solute-solvent interactions between ground and excited state, and differences in excited-state lifetimes resulting in larger contributions to the photochemical reactivity and absorbance mismatch. Critically, we submit that the microenvironment effect is likely translational to photochemical reactions other than photocycloadditions where a strong mismatch is observed, including those that result in intermediate species or further reactions, such as photoinitiated polymerisations. However, these reactions are expected to be more complex to investigate and quantify due to more complex product formation mechanisms.