The reason you are seeing that is because the RAW converter is not trying to match the JPEG. It is trying to provide a faithful rendition of the scene. Also, camera companies do not provide information about their in-camera settings, so RAW decoders generally don’t have the information necessary to match them. A simple example is to shoot a picture as Monochrome in the camera. The RAW will be in color. Some companies do try to match the JPEG more closely but they also may be getting information from the camera companies, that I do not. If it’s always darker, then you might consider making a camera preset (the user manual discusses how to do that).