Fully microscopic many-body models are used to calculate the radiative losses in GaN-based light emitting devices. It is shown how simpler models under-estimate these losses significantly. Using the high accuracy of the models allows to eliminate the corresponding loss parameter (B) and its density- and temperature dependence from the space of parameters that are used to fit effuciency data. This allows to study the dependencies of the remaining processes with high accuracy. Using this model, it is show that many processes that have been proposed as causes for the efficiency droop either have wrong dependencies, magnitudes or require unreasonable assumptions to explain the phenomena in general. The most plausible droop model appears to be a combination of carrier delocalization at very low temperatures and pump powers, density- activated defect-recombination at low to medium pumping and injection/escape losses at strong pumping.