The magnetization curves of YBa2Cu3O7-delta crystals are widely observed to exhibit a pronounced low-field minimum in the temperature range 30 K < T < T(c). We demonstrate that these magnetization anomalies correlate with the c-axis lattice parameter, and hence with the specific oxygen deficiency delta. We interpret this low-field feature in terms of a field-induced decoupling of regions of oxygen-rich material by boundaries of oxygen-poor material, a phenomenon reminiscent of the behavior in granular superconductors. A "phase diagram" that demarcates the multigrain onset as a function of temperature and delta is then constructed. The shape of the "phase boundaries" is described using a percolation model. This analysis demonstrates that magnetization is a very sensitive measure of crystal quality.