Solution : https://service.sap.com/sap/support/notes/308461 (SAP Service marketplace login required)
Key words : 
alternative items slightly deviate, additional key words co01, header quantity, process order, component quantities, expected values, prerequisites make, solution carry, program corrections, production
Related Notes : 
       
| 152477 | Rounding error in qty changes in production order | 
| 117321 | CO01,02: Rounding error for component quantities | 
| 79149 | Alternative UoM: Incorrect adjustment of comp. qties |