The theoretical average line-of-sight velocity dispersion along the radius of a spherical galaxy or cluster of galaxies tends to a variable value. Statistical lensing studies often take into account only the asymptotic value, either close to the center or at large distances, rather than the full variation: this results in significant effects in lensing properties depending on the mass density models of the lensing systems. Magnification probabilities by clusters of galaxies modeled by four well-known density profiles are calculated in this paper and the uncertainties from the variable velocity dispersions are shown