Realized Gain/Loss vs Rounding Suspense Error Entries

I'm trying to understand when a realized gain/loss vs Rounding Suspense Error entry should be generated. We're on 9.1, 8.5.2 tools and have the following scenarios for Accounts Payable:

Create a voucher in currency GTQ (base currency of the BU is GTQ) and post it at one exchange rate and then update the exchange rate prior to paying it. Pay the Voucher out of the bank account in USD. In our system a Rounding suspense entry is generated for the difference in the paid amount base vs the amount calculated for the accounting entry (paid amt * exchange rate of USD to GTQ).