Peter, they either have the date wrong that Franz Behrens was killed around Cassino, he was killed by air attack, or he died of sickness because it shows he died/went missing on 03-03-1943 and the Allies did not invade Italy until July 9, 1943 in Sicily.
http://www.custermen.com/ItalyWW2/History.htm
The Volksbund sometimes has the date wrong but then we are usually talking about a day not months; I think we can presume that this Franz Beherens died from an accident or so and after the war also ended up being buried at the German wargraves cemetery in Cassino.
P