Why does the supplier audit fields include SetID and Vendor ID if these cannot be changed?

The Supplier audit fields to choose from includes the Supplier SetID and the Supplier ID.  However, once you create a supplier under a SetID with a specific Supplier ID those fields cannot be changed.

The record and field names behind these are:

Record is PS_VENDOR

Field names are SETID and VENDOR_ID

On the supplier audit template, supplier setID and supplier ID can be checked to be audited.  However, on the supplier Identifying Information tab/page, these two fields are key fields and (understandably) grayed out.