[fingerlocks]

Source? The numbers correspond to the USPS distribution centers and carrier routes. If the numbers are changing that would imply an increase in zip code subdivisions, making each zip code a better address predictor for a given individual.

[bluGill]

https://faq.usps.com/s/article/ZIP-Code-The-Basics

I don't know how to link to the correct question but there is why did my code change.