Using date instead of period in the Balance by field when calling the report.
Using incorrect value in the Rate field (current/historic).
The inventory balance has not been transferred in the Period account register (PAR).
Transactions posted manually against the inventory accounts in the general ledger.
Check this by writing ATRLIST/1xxx in the program field to list the GL entries for this specific account.
To filter the list to a specific period and to manual postings, write this command in the filter field: PERIOD="YYYYMM",TRANS="*MAN*"
Minor deviations may be caused by agio.