Output VAT difference due to foreign exchange loss on account No.********* of ********* comp code

Hi Friends ,

Sales invoice : 04.04.2014

entry :                                        USD                       GBP

                  customer a/c dr        4368.00                 2626.04

                   To vat output a/c      728.00-                   437.67-

                   To other income       3640.00-                 2188.37-

Credit note :  04.07.2014

                                                USD                        GBP

                VAT out put tax        728.00                       424.06

             foreign exchange loss     0.00                       13.61

              Other income             3640.00                     2188.37

                     To customer a/c  4368.00-                     2626.04-

Based on above entrys  Why foreign exchange loss only for VAT output account not for customer account ?

Please let us know any configuration error and please guide me this is urgent issue.

Thanks for understanding

Regards,

Seshu