Vielleicht liegt's daran:

AS/400 Performance Management Version 3 Release 1 (April 1995)

... Other systems may process other numeric field types more efficiently. For example, System/36 uses zoned decimal arithmetic. Since the System/36 RPG compiler on the AS/400 (CRTS36RPG) does not generate packed decimal fields, you must use the CRTRPGPGM to get packed decimal field code generated for running on an AS/400.