In the transaction, you specify the source and destination accounts, the destination amount and currency, and the source currency. If you specified the source amount, the transaction would fail if the network state was even slightly different from the state it was in when you formed the transaction. So instead, you specify a maximum amount you are willing to pay to deliver the destination amount. The network computes the minimum amount to make the payment with the paths specified. So long as it is equal to or less than the maximum source amount, the transaction goes through. The amount you pay is the actual amount needed.

So it's not that you transfer a little more, it's that you allow the transaction to transfer a little more should that be necessary.