The reason you will find a German address in the WHOIS information for your German .DE domain, is because of the so-called 'Trustee Service' that is set for this domain.
It is compulsory for .DE domains to have an administrative contact address in Germany. When you are registering a .DE domain you can choose if you need to use the Trustee Service. In case you have your own address in Germany, the Trustee Service is not necessary. In the case you do not have an address in Germany, you can use our Trustee Service that automatically replaces your address with a postal address in Germany to ensure that your domain complies with the rules. This is called the 'trustee' of the domain.
You will find the following information in the WHOIS information:
Sandra Frisch Neunkircher-Str. 43 66299 Neunkirchen
TransIP obtains the postal address from this party. In case you need the Trustee, a small fee will be charged. In case that you will use your own German postal address, no additional costs are charged.