We investigate the large differences classically found in literature between theoretical and empirical results in the optical characterization of macroporous silicon photonic crystals. Mainly, we study the impact of two optical absorption mechanisms, free carrier absorption and scattering. Optical measurements with different membranes and previously reported data have been compared with numerical simulations. Results show that free carrier absorption is irrelevant for the used bulk doping. Nevertheless, the implanted N+ ohmic contact imposes a high absorption in the long-infrared region, leading to a 40 % decrease in transmission. Concerning scattering, we demonstrate that error/deviations in the etching of the photonic crystals –such as branching or spiking–, with sizes comparable to the modulation radius, are responsible for Lambertian light trapping. This absorption mechanism is dominant for wavelengths shorter than the bandgap ones, regardless of the horizontal periodicity used, due to the size dependence between error/deviation and modulation radius.