German also has hardening of end consonants, and not only in words but in syllables. That's one reason. Then, I suppose that very often German speakers will mix up the c and з sounds. In German, c cannot be in the first position of a word, for example (apart from forewign words, such as service, of course - but even then many people will use a voiced s.). In other places it's the other way round. And in German p, t and k are always aspirated, it's actually pretty hard to not aspirate them that much.

All in all, there is only very little agreement between the phonemes used in German and in Russian.