Solution : https://service.sap.com/sap/support/notes/43551 (SAP Service marketplace login required)
Key words : 
additional key words transaction va41  va42billing schedule fplt fplapricing, symptom roundoffs occur, field komv-kawrt, billing schedule, condition values, monthly prices, basically determined, decimal places, condition base, database conversions
Related Notes : 
       
| 139660 | Interval scales in the billing document | 
| 92655 | Billing plan: billing yearly prices on monthly basis | 
| 60665 | Condition basis in statistics for interval scales | 
| 60035 | Display of condition/variant factor | 
| 53705 | Subtotal: Cond.base value and amount incorrect | 
| 47416 | Increase scales are not determined |