Hi Vineet
If you have change logs enabled for TCURR, please check when the FX rate was maintained. Suppose you did your payment processing at 10 AM and exchange rate was maintained at 10.30 AM and you posted BRS at 12 PM, there could be FX differences, if the exchange rate was different.So that could be the reason of difference.
Another reason could be that while processing payment, say by a manual transaction like F-53 or F-58, you have a chance to enter exchange rate manually. So if your user has entered the exchange rate manually there, it can also result in FX difference.
Thanks & regards
Sanil Bhandari