Solution : https://service.sap.com/sap/support/notes/1041753 (SAP Service marketplace login required)
Key words : 
call transaction faglb03, general ledger account, display data related, fiscal year variant, relevant support package, created special periods, 12 'normal' periods, special period, displayed correctly, terms falgb03
Related Notes : 
       
| 1407784 | FAGLB03: Special periods nt dsplyd crrctly (addtional chars) | 
| 1047254 | FAGLB03: Drilldown using 'Total' incorrect with periods < 12 | 
| 672255 | Shortened fiscal year/changing fiscal periods |