The asymmetry of (GaAs)(1-x)(Ge-2)(x) band gap bowing, as a function of Ge concentration, is investigated by ab initio calculations. We show that the nature of the chemical bonds violating the octet rule in such alloys is affected by the local geometry: the switching of the cluster type between Ge to GaAs domain at Ge concentration of about 0.3 plays a fundamental role in the final asymmetry.