Some exchange using this method.

For the deposit process, they are using a payment gateway from a third service not using their own wallet. In a regular deposit system, you don't need to fill the amount you want to deposit in here you are filling the amount of money you want to deposit and the third service has counted the estimation of currency you are choose need to be sent.

No need to confused, just send the fund exactly the same amount on the invoice. Deposit system like this, If you send less amount then the invoice will be updated with the latest update (The rest amount you should send).