As you say, Numisdata has two different data publication scenarios: Web and Nomisma format.
Web (web_numisdata_default) define full publication of fields with complex data as legends, bibliographic references, etc and lots other things that are not in Nomisma definition. So you can use it to build a rich web page of your data.
Nomisma: define the "export" data to build a compatible Nomisma RDF, but it's limited to Nomisma definition (you will lost some fields and definitions), so, use it if you need to share data with other Nomisma projects as OCRE. You can thing in these more in "export" way than publication, but it's defined as publication instead export because the websites need to use it to link web with RDF "equivalent".
I hope it's clear.