I currently have an OSM "stack" server (Postgres/PostGIS, Mapnik, OpensLayers) working in a development environment, with almost all the South America countries loaded on the database. Since the applications I'm targeting uses geocoding extensively, I made a test deployment of Nominatim.
While testing, I discovered that almost all searches on Buenos Aires (where my company resides) didn't return a "house number" or something like the "between numbers" of Google Maps. The official Nominatim site didn't return that data neither. But, using the Nominatim service of MapQuest, there is always a house number on the returned XML.
Since MapQuest theoretically use the same data and tools, why they can show that information? Are they using another source of spatial data?