Securities to know position entries

Dear friends,
I need to know about position accounting entries(SARUSD140160001/USDSAR140160001) for Securities FCY transaction. Whenever i buy FCY security, i can see an position accounting entry and this create difference in Lcy amount on comparing with P/L amount, which will raise entry in internal account.On analysis, the position accounting entry has different exchange rate. May i know, from where this exchange rate has been obtained. Please refer the below example,

MID.REVAL.RATE IN CURRENCY(SAR) - 3.7504
P/L ENTRY EXCH RATE - 3.750400000
SARUSD140160001 EXCH RATE - 3.75

I need to know, why the system update different exchange rate for account 'SARUSD140160001'

DATE ACCOUNT CUST COMPANY VALUE DATE CCY FCY AMT LCY AMT
02 JUN 2013 USD140550122 BNK 02 JUN 2013 USD 0 -0.01
02 JUN 2013 SARUSD140160001 BNK 02 JUN 2013 SAR -280.35 -74.76
02 JUN 2013 50011560682 1560 BNK 02 JUN 2013 SAR 187,180.35 49,909.44
02 JUN 2013 PL 52-603 1560 BNK 02 JUN 2013 SAR -280.35 -74.75
02 JUN 2013 LIVE BUY 999999 BNK 02 JUN 2013 SAR -186,900.00 -49,834.68
02 JUN 2013 USDSAR140160001 BNK 02 JUN 2013 USD 0 74.76




Kindly reply, if anybody knows details.