Do German tour operators market Cornwall as a destination in its own right? I expect many will operate tours around the South of England, in which case you may find that a large number of tourists arrive and depart via London.
Of course, Cornwall is an attractive destination by itself, but it may explain why the German market from Newquay has not developed in the same way as it has in the Channel Islands, where there are flights from Germany, Austria and Switzerland in the summer.