Yes, I know that is the current way they work (...and I said it in my comment?)

My point is that postal codes aren't "supposed" to work any particular way. They work in what ever way they are specified to work at any particular point in time in any particular place.

Zip codes in the US are related to delivery routes. Prior postal codes in the US were not.

In the US they were always tied to routes. The first number gives a set of distribution centers, and the second two numbers where specifically to go. If you look at a map of the first number, you'll see sharp lines separating the states. It'd be hard to verify historically, but if you look at zip codes near the border for, say, 89439, several addresses on the California side use the 89439 zip code.

It'd find it difficult to believe that when the system was originally created, mail delivered by a Nevada post office to a physical location in California had a zip code starting with a 9 rather than an 8.

Zip codes were always tied to routes. Postal codes in the US pre 1962 were not.