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The US GeoCoder and US Zip9Coder both generate CensusId information, but they are not the same all the time. Does anyone know what that piece of information is based on and why it is different in some cases?
The Censusid is based on US Census geography - specifically census tract / block info. That geography doesn't map one to one to postal geography (such as zip9). So, sometimes the relationship will be the same, and sometimes not. The zip9 relationship is less likely to be accurate of course.
You can think of it this way: If you assign an address the default zip code for a city, sometimes it will be the same as the true address zip code, and sometimes it won't be since the city likely has multiple zip codes.