If the dealership took on the responsibility for installing the diff cover, then it's on them to make this right. If they did not explicitly follow instructions, my guess is that they over or under torqued the cover bolts and the gasket was either distorted or not fully seated against the diff housing respectively. It could also be that they didn't adequately clean the mating surface on the diff housing which would make it more difficult to get a good seal due to the irregular surface.
To be honest, I let the dealership touch my vehicle as little as possible. When they do, I back off and retorque to spec any fastener associated with a seal. Lastly, I always check fluid levels since it's been my experience that my local dealership service department likes to overfill or underfill. Unfortunately, it's not commonplace for all techs to have a sense of professionalism and/or give a damn about quality of work. Most are just trying to get from one job to another as quickly as possible.
