ZIPs don't represent areas either. They represent routes, so a collection of paths on a road map is bundled together and named with a ZIP. If you built a new house in between towns, whichever post office decides to serve it would extend one of their lines to go past your house, and then you are assigned that ZIP code. Since it's all based on their convenience, they don't pay any mind to things like city limits or even in some cases state lines.