Postcode mystery

[Waterwolf]

Why is that whenever I get a pre-addressed envelope through the post (to send a reply back), the postcode on the envelope is often different from that on the letter that came with it -- even if the rest of the address is the same?

[KEJ2000]

I once briefly worked as an office temp. in the administrative headquarters of the U.S. Postal Service, in the USA. It occupied one floor / level of an office building, but because such a large quantity of mail was generated there, our floor had its own Zip Code. My guess is that some businesses that occupy two floors in the same office building could have a different Zip Code (in your case, Postcode) for each floor. The Sending department may be on one floor, while the Receiving department is on another floor.

[Hippy]

It is possible to have two postcodes for the same address. One is the usual one, conforming to geographical location, and the second a so called "Large User" postcode. These are for people like DVLC Swansea, certain broadcasters, and other large organisations. These Large User postcodes often contain 99 in the first part (NE99 2BB for example).