We propose an explanation of an asymmetry in the occurrence probability of the Io-dependent Jovian decametric radiation. We find that this asymmetry arises because when Io is in the northern part of the torus, more intense Alfv¿n waves are generated propagating southward than northward. These waves then cause the excitation of decametric radiation in the northern ionosphere after reflection from the southern ionosphere. The asymmetry then results from the propagation time of the Alfv¿n wave and the bending of the magnetic field (Alfv¿n wing) along this trajectory. The ray paths of the decametric radiation are calculated using a three-dimensional ray-tracing program in the Jovian ionosphere. Variations in the expected probability plots are computed for two models of the Jovian ionosphere and global magnetic field, as well as for several choices of the ratio of the radiated frequency to the X mode cutoff frequency. The dependence of the occurrence probability on the Jovian declination of the earth is also discussed in the context of this model.