Currency gain loss question

Hello,

I have a question about currency gain loss calculation logic of applying customer entries,

In the apply customer entries page;

-The upper part:
Amount is negative, currency blank (payment etc.)

-subpage part:
Amount is positive, currency is not blank (invoice etc.)

In this scenario system calculate currency gain loss, but if I move the invoice to the upper part then system doesn't calculate currency gain loss.When I check the code, ApplnExchangeAmtFCYToFCY (Currency Exchange Rate Table) function is called and this logic is executed.

Do you have any idea why this logic works like that and what are the effects of it?

Thank you.