Dear Omer,
You may find the difference between the ECC and BW report as in ECC the netduedate calculation is different. (for BR type its based on the invoice reference netduedate).
You can do the below to reach to the exact issue
1) Take one customer for which you have the difference for 1 to 30 days. (As on one particular date for eg 30.09.2014)
2) Navigate by document number and netduedate
3) Run the FBL5N for the same customer ( as on the same date as you run the query 30.09.2014)
4) Now filter the net due date which falls between 30 days for eg 31.08.2014 - 30.09.2014
5) Now analyse which document is giving the difference between BW and ECC ( Mostly it will be a BR document which will have difference between ecc and BW)
Please let me know your feedback
Regards
Gajesh