Postal areas are operational
ZIP Codes are designed to support delivery operations. Their shapes and coverage reflect routes, delivery points and postal facilities rather than a single consistent geographic rule.
Not every code is a polygon
Some ZIP Codes are associated with PO Boxes, unique recipients or delivery routes and may not correspond to a conventional residential area that can be drawn as a simple polygon.
Different map products
Mapping providers and research datasets may approximate ZIP Code geography differently. Census ZCTAs, for example, are statistical approximations and are not official USPS ZIP Code boundaries.
Choose data for the task
Use official postal address tools for delivery validation, Census geography for demographic mapping, and clearly documented datasets for spatial analysis. Always explain the limitations of the geography you choose.