Not only a zip code can span cities, a single city/zip can span multiple tax authorities e.g. parts of Redmond, WA zip code 98052 are under Regional Transit Authority and others are not.
I think that pretty much any assumption we're making about strict hierarchy are bound to be broken at some point.
I worked with zip codes on a project. Zip codes are a nightmare. They cross everything. There are zip codes which cross state-lines. Zip codes which are not a single contiguous area of land. So many issues.
I feel your pain. It turns out there are even a group of ZIP codes that apply only to specific businesses, buildings, or even people (if you're important enough).
I think that pretty much any assumption we're making about strict hierarchy are bound to be broken at some point.