Solution : https://service.sap.com/sap/support/notes/210114 (SAP Service marketplace login required)
Key words : 
additional key words rounding, purchase order transaction me21n, purchase order quantity, enjoy purchase order, attached program correction, transaction me21, order unit, rounding profile, releases 4, prerequisites
Related Notes : 
       
| 316707 | ME21N/ME22N:Rounding values not taken into account | 
| 300252 | ME21N/ME22N:Repeated rounding after incorrct roundg | 
| 207774 | ME21N order unit not taken from PReq |