Differences between balance general ledger and balance of payments to be processed in ISU

Hi experts,

I find small differences between a general ledger account's not assigned payments in R3 with a total balance of not assigned payments in ISU.  Differences on this general ledger can occur because incidental reversals in the returns lot sill have to be posted manually in order to process the payments. In this case we see that there are not reversals to be processed in the returns lot.  

There are daily smaller differences to maximum € 9,00, but in most cases smaller then € 1,00.

I also checked reconciliation key but there is no error in it. The transfers to general ledger are completed successfully without any error.

What could be the possible reason for these kind of unexplained differences in general ledger? 

Thanks and Regards

Dushyant