Solution : https://service.sap.com/sap/support/notes/152477 (SAP Service marketplace login required)
Key words : 
additional key words co01, rounding errors occur, type floating point, header quantity, production order, component quantities, incorrect quantities, solution correct, calculation, co_mk_resb_amount
Related Notes : 
       
| 308461 | Rounding error in quantities of alternative items | 
| 117321 | CO01,02: Rounding error for component quantities | 
| 93354 | Incorrect component qty when changing header qty |