This is because the "pienalueet" are *statistical* divisions, while "kaupunginosat" are *zoning/planning* divisions. Pienalueet are subdivisions of tilastoalueet (statistical areas) which are subdivisions of suuralueet. Kaupunginosat (henceforth "districts") are a completely different division.

The districts are only for urban planning and statistics are not collected from them. Estate numbers (kiinteistönumero) contain the district number in the second part of the four-letter code; in areas that have been zoned, elsewhere the village number (4xx) is used. For example Ristikivi has both "179-25-xx-x" (25 is the district number) and "179-402-xx-x" (402 is the village number for Keljo)