OpenSim and MuJoCo are two physics engines with their similarities and dissimilarities. The conversion of an OpenSim model to MuJoCo is not an easy process, as the models are built differently and their specifications are defined differently. This means that there doesn't exist a one-to-one mapping between a model implemented in OpenSim and a model implemented in MuJoCo. An OpenSim model converted to MuJoCo is only an approximation, although we try to optimize the converted models parameters such that it behaves similarly to the OpenSim model.
We welcome any suggestions and improvements to the optimization and conversion process, particularly if they come in the form of a GitHub pull request.