Selective excitation photoluminescence measurements performed in CdSe/ZnSe superlattices with an embedded ${\mathrm{Zn}}_{1$-${}x}{\mathrm{Cd}}_{x}\mathrm{Se}$ quantum well reveal the limitation of the mobility edge concept to describe the dynamics of localized excitons in this system. Both localized and extended excitonic states are found to coexist in a certain energy range. We suggest that this behavior is governed by a macroscopic band-gap modulation imposed on the microscopic random localization potential.