How are numbers matched in OSM Nominatim Geocoder search?

Same question asked in http://stackoverflow.com/questions/31903162/how-are-numbers-matched-in-osm-nominatim-geocoder-search?noredirect=1#comment51750986_31903162

Here is one things that I cannot make sense of how Nominatim handles numbers (at least in Barcelona):

Searching for Passeig de Gràcia and appending numbers 38 or 1, or 20 all returns the same set of results. I don't know where it's matching '38', or '1', or '20' because they are not in the results as fields. Searching for Calle Mare de Deu dels Desemparats 18 is not returning any result but removing 18 returns 1 result. Why does the service appear to be lenient to the number search parameters of Passeig de Gràcia while strict with Calle Mare de Deu dels Desemparats?