When you instigate a payout (A) from your MangoPay trustee account to your normal business account, you can specify the amount of the payout you want yourself (B).
Please note that the credit amount which can be paid out is made up of the total of all transactions since the last payout less the 5% guarantee retention. So there will always be a difference, because of the guarantee retention. There will also be a difference because you yourself determine the amount to be paid out, and you do not necessarily have to choose the maximum sum payable.
The payout receipt, which you can download by clicking on the right arrow next to the payment amount for the payment concerned under Billing> Summary shows the payment amount, the guarantee retention and the initial and final balance in your trustee account: