Inventory managment: 2LIS_03_BF - beginning $amount in BW are not in sync with ECC

Hello experts,

I have an issue with 2LIS_03_BF and 2LIS_03_BX loads for the inventory management. My understanding is:

The beginning amount in BW for each month = the previous month’s ending value in ECC( Tcode MB5B).

I checked the data after all the initial loads, for the current month (sept/2013), BW data is in sync with ECC for all data fields including the beginning amount.

However for all the previous monthes (about 13 months), the beginning amount in BW is not in sync with ECC.

So I am guessing the 2LIS_03_BX load is good since it is loading the current stock, and the issue should be in 2LIS_03_BF load.

Although the key figure in BW for the beginning amount = 0RECVS_VAL - 0ISSVS_VAL,but from some research it indicates the beginning value is calculated backward: for an example, the beginning amount of 8/2013 = 9/2013 –(+) material transactions during August/2013.

My questions:

1: how exactly the beginning amount of previous month is calculated?

2. How to resolve our issue?

Your help is greatly appreciated!

Thanks,

Jun