In my understanding:
LNAB means using the last name as it is recorded on the record nearest to birth.
Since Norway Project has standardized on using the farm name as last name (if the individual does not have a hereditary name) the LNAB “rule” means using the farm name as it is recorded on the record nearest to birth.
The Norwegian digital archives (https://www.digitalarkivet.no ) does not standardize farm names. Instead, when transcribing into searchable format, they transcribe the farm name as it appears in the original record. This avoids introduction of errors.
That said, Olaf Rygh’s listing of farm names ( http://www.dokpro.uio.no/rygh_ng/rygh_info.html ) is a useful tool. It provides a snapshot of the main Norwegian farm names at a point in time (~1950) & provides farm name variants. For example it tells us that the farm “Fjellstad” has also been written: Felldstad, Fiællestad,Fieldstadt, Fielestad, Fiellestad, Fillestadt, Fjellestad, & Fjelsta. It is useful in figuring out what variants in recording might exist.
This is hardly unique to Norway. In 17th century New England, the spelling of last names changes as the recorder changes. It is not unusual to find 7 children to the same couple with 3 different last names (e.g., Harradan, Harraden & Haradan).
Best wishes on your searches!