Is it possible your API results are simply showing the GMT date?
Pretty sure the API is going to return GMT (Zulu) because all databases require that you render to your locale (whatever that may be).
As far as other mysteries, fields can be set to map a common time for all users in Airtable’s formatting options. I’ll bet that’s why you see the one hour diff on some tests. As such, users in two different time zones could see the same timestamp on a record.