We theoretically and experimentally analyze the angular selectivity curves of nonuniform gratings recorded in a photopolymerizable silica glass. Due to the rigidity of this material, shrinkage would not cause fringe bending, so we propose that the coupling of the beams at the recording stage (two-wave mixing) produces the resulting nonuniformities. Nonuniform transmission holographic gratings can be analyzed in rigid media using the coupled-wave theory but introducing the results obtained from the two-wave mixing treatment and taking into account the absorption and the nonlinear effects produced by the mechanism of photopolymerization and diffusion. Good agreement between theory and experience is obtained.