Unless the field is on a different segment of the business object, then I will call it bug or bad design by SAP. You know like using BDC for inbound vendor master creation via IDOC on ECC 6.0 or not having any real-user exits in the ADRMAS idoc type.
That being said I think Luis was wrong about SAP Press authors being part of the kind souls group. I would instead add them to the "tortured souls" group. I really can't complain about the documentation gap today for SAP solutions. You really never learned SAP R/3 unless you had to use physical CD documentation ( what's help.sap.com ) and the red & white ABAP book.
Take care,
Stephen