This may vary depending on where in the US you are, but typically as part of the development process addresses are assigned by the city/county assessor around the time the plat is recorded. I don't think it's so much a case of 'validating' as getting updated data, just like your geocoding vendor. It sounds like the post office has more recently updated their data than your vendor - and both probably get it from the county or another middleman who gets it from the county. My local county publishes assessor updates every two weeks, while others nearby are only quarterly or offer a subscription service where you basically have access to their current data at any given time.
In the specific case I remember working on (as a designer of the subdivision), I was working with someone at the city to get street names approved with emergency services, then they had a GIS person do a first pass at addressing, which was then reviewed by emergency services and someone with the (local I assume) postmaster, some revisions made (ie, address this corner lot off this street instead of that one, or add some extra numbers in case this lot is ever subdivided, locations of mailboxes, etc.), and then it was all approved and recorded. I assume the assessor records the official record, and those updates are given to the postmaster to enter into their system.