Mortality rate is very dependant on age and health. The initial infection in Italy was in an hospital and so the initial propagation was in older population and in the medical staff which have a lot of contact with old people and people with bad health.

On the other side, people in Germany who gets the virus were very often young people travelling to Italy and they contaminate their young and healthy German friends when they come back in Germany.

This is probably enough to explain the initial difference in mortality.