Birefringence can contribute to x-ray resonant Bragg diffraction and likely explains recent novel data collected on CuO. We prove these statements using ab initio simulations which reproduce the experimental polarization effects quantitatively. We show that an unrotated polarization signal---ruled out in resonant magnetic scattering within the electric dipole approximation---arises from the dynamic change in polarization inside the material. We are able to reproduce all the related behavior with circular polarization and its dependence on the angle of rotation about the Bragg wave vector. We provide a tool to disentangle the various physical origins of the polarization rotation, providing a more complete understanding of the illuminated material.