Exchange Rate of sum of Customer Balance for two Company Codes

we have a Customer we need to know the Balance in Forgien Currecny and Local Currecny (USD is the Forgein Currency and EGP is the Local Currecny) for the Two Company Codes we are using FBL5N but there is something werid

In the First Company Code

Amount in DC Curr. Amt in loc.cur. LCurr >>>>>>>>>>>>>>Rate is 6.41

421,225.04- USD 2,704,228.62- EGP

In the Second Company Code

Amount in DC Curr. Amt in loc.cur. LCurr>>>>>>>>>>>Rate is 6.70

490,188.91 USD 3,286,600.67 EGP

Both Company Codes :

Amount in DC Curr. Amt in loc.cur. LCurr>>>>>>>>Rate is 8.44!!!!

68,963.87 USD 582,372.05 EGP

what i need to understand how come the average rate for the two company codes is 8.44 Although the rate for the first company code is 6.41 and the second is 6.70 which means that the average for both is 6.55 not 8.44