Recently Clancy reported a Monte Carlo simulation that reproduces the tendency, observed by Mulhaupt and co-workers, for demixing of melts of isotactic polypropylene (iPP) and syndiotactic polypropylene (sPP). The short-range intermolecular interactions in the coarse-grained chains in the simulation are controlled by a rotational isomeric state model for PP that was described by Suter The intermolecular interactions of the beads are controlled by a discretized representation of a continuous Lennard-Jones (LJ) potential that describes the interaction of two molecules of propane. The work reported here reveals that the outcome is sensitive to the truncation of this LJ potential. A weak tendency for demixing of the 50:50 iPP:sPP melt is seen if the LJ potential is truncated so that only its repulsive part is used in the simulation. Inhomogeneity is enhanced if the truncation of the LJ potential is shifted to larger distances, so that the attractive branch of the LJ potential is incorporated in the simulation. This observation in the simulation leads to conclusive identification of the mechanism responsible for demixing of the 50:50 melt. It also implies that the same mechanism may affect the miscibility of melts of other vinyl polymers in which the chains differ in stereochemical composition.