Places   (Help with places, streets and buildings in Wigan.)

The reason is simple. The postal service is operated by the Royal Mail and they set the postal areas. The telephone system is operated by British Telecom and they set the area codes. The UK is operated by The Government and they set the local government areas.

You'd may as well have asked "why does a Golborne house get Stockport milk delivered by a Haydock Milkman who has his float serviced at a Westhoughton garage"!