Solution : https://service.sap.com/sap/support/notes/84112 (SAP Service marketplace login required)
Key words : 
open schedule line quantity incorrectly, affects open order credit values, scheduling agreement header    processing type, additional key words sis, vbuk-vbeln    i_item_number, vbup-posnr    i_type, vblbvbthe source code, function group v45l, function module rv_schedule_check_deliveries, function module sd_delivery_cumulative_determ
Related Notes : 
       
| 204130 | SIS update: Collective note | 
| 191939 | Component supplier: MCVBEL incorrect for VL04/VL10 | 
| 130342 | Incoming orders not deleted when doc. deleted | 
| 129470 | Incoming orders for component suppliers: Alternative update | 
| 125763 | SIS: Collective note - component supplier | 
| 125700 | MCVBEL-VSMNG_L and MCVBEL-VSWRT_L not filled | 
| 98431 | SIS - open quantities incorrct f. corrction delivry | 
| 95747 | Incorrect proposal of cumul. go-ahead quantities | 
| 92361 | Sched.agrmnts open qty/credit value in sched.lines | 
| 81335 | Incoming order for sched. agreements not correct | 
| 77319 | Deliv.sched. processing: incorrect delivery index |