What changes when a country redraws its administrative borders
When a region merges, splits, or gets renamed, every address and every dataset referencing the old boundary needs to be updated, and that update rarely happens everywhere at once.
Tag
#administrative-boundaries
Why postal code boundaries and administrative boundaries differ
A postal code area and a city or county boundary are drawn for entirely different purposes, and they frequently do not line up, which trips up anyone who assumes they should.