Solution : https://service.sap.com/sap/support/notes/117321 (SAP Service marketplace login required)
Key words : 
additional key words co01, change header quantities repeatedly, change header quantity, component quantities deviate, process order, expected values, solution correct, source code, implemented version, implemented
Related Notes : 
       
| 308461 | Rounding error in quantities of alternative items | 
| 152477 | Rounding error in qty changes in production order | 
| 79149 | Alternative UoM: Incorrect adjustment of comp. qties |