As a fellow German please remember that these cases are reported more than 'normal' rape cases. It's a simple case of sample/selection, the most visible cases seem more frequent, thus more people focus on them, thus they get reported more. Think of the Russo-German girl in Berlin, where it turns out it was all just a false report (girl ran away from home and didn't want her parents angry when she came back, so claimed to have been abducted...), This was played ridiculously out of proportion, largely because Russian-funded media threw itself behind this case, even long after it was debunked.

Tl;dr: what is salient seems more frequent. Just like people think shark attacks and plane crashes are much more frequent than they are.