Sudheer,
Did you check the schedule lines of the sales order line item?
As i said if you found two schedule lines( ie one for 70 qty in backward schedule line system confirms and remaining 30qty system confirms in future date by calculating in foreword scheduling)
Then it is standard behavior only, it is not strange.
If the line item has got only one schedule line line that is for qty 70 even then system is updating the sales values into FD32 for 100qty then you need do debug it.
thanks,
Srinu.