I understand this.I saw in Streetlights lamps with location like "11/15" where for sure no №13.In this area it's a common thing.Closest №13 is http://www.openstreetmap.org/way/1110390172
I only know here is not enough buildings to interpolate properly.But I believe there can be more than one way to enumerate buildings (that's why I've stopped interpolating Streetlights' housenumbers) and this is just a tip for observers