In many-component semiconductor heterostructures photoluminescence (PL) is strongly affected by the disorder potentials caused by compositional fluctuations. We present an experimental study on the temperature-dependent PL in the Ga(N,P,As)/GaP quantum well which indicates the intriguing feature that the energy scale of the disorder decreases with increasing concentration of the fluctuating compositional component (nitrogen). This effect strongly suggests that the impact on the band structure and the effective mass both decrease as the N concentration increases. This conclusion is supported by theoretical estimates using the analytical theory of compositional fluctuations in mixed crystals.