Solution : https://service.sap.com/sap/support/notes/78687 (SAP Service marketplace login required)
Key words : 
additional key words co40, attached source code corrections, planned order quantity, planned order, order quantity, production quantity, component quantities, production unit, alternative units, prerequisites solution
Related Notes : 
       
| 174538 | CO40: Incorrect order quantities in collective order | 
| 113906 | Incorrect scrap qty when converting planned order | 
| 100901 | COR1,COR2: Chnge header qantity=>cmpnent quantities | 
| 94053 | Incor. component qty when converting planned order | 
| 81121 | Rounding order quantity when converting plnd order | 
| 77187 | Computation of quantities for material components | 
| 73033 | Component quantity in production orders |