Searching for objects in centre of the German city of Bielefeld, I find many results listed as though they were located in the quarter of Quelle, which is a suburban area SW of the city centre. Try the University for example: <http://www.openstreetmap.org/search?query=Universit%C3%A4t%20Bielefeld#map=18/52.03775/8.49329>, and to understand the problem you can click on the first result <http://www.openstreetmap.org/way/35579376>, zoom out and compare with a map for Quelle at the German WP article <http://de.wikipedia.org/wiki/Quelle_(Bielefeld)>. In terms of administrative divisions, Quelle is part of the district of Brackwede which is not even adjacent to the district of Schildesche (which contains the university) or Mitte (centre, which contains many more hits that Nominatim outputs as “Mitte, Quelle, Bielefeld” if you search for them). In those result lists, the name Quelle always appears between the district (Bezirk, the first administrative subdivision level in Bielefeld) and the city name (Bielefeld). I’ve made a few changes to OSM using the Java as well as the web-based editors in the past, so I’d like to understand what causes the problem, not just get someone to fix it for me. So, could you take a look and explain? Thanks.
There are some topics about wrong results of the Nominatim search engine at http://help.openstreetmap.org.
Maybe you can find some more hint there.
Or ask at the German subforum here, if not done yet. There are at least specialists about administrative boundaries.