Solution : https://service.sap.com/sap/support/notes/115011 (SAP Service marketplace login required)
Key words : 
i_prodcost        rc27x-flg_sel            space         xthen implement, additional key words ck11, additionally implement notes 100996, rounding errors occur, function module 'co_mk_resb_amount', source code corrections, product costing, usage probability, usage probabilities, problem occurs
Related Notes : 
       
| 320182 | Assembly scrap in the material cost estimate | 
| 100996 | Alternative item with usage probability in costing | 
| 89557 | Cost est with quantity struct: Rounding indicator 1 | 
| 79149 | Alternative UoM: Incorrect adjustment of comp. qties | 
| 74859 | Costing: Alternative items with wrong quantity |