TransferWise uses the real, mid-market exchange rate. The exchange rate is locked in when you set up a transfer, for a specific amount of time. If your money reaches us later, we'll convert your money with a different rate. This might complicate things if your recipient needs to receive a specific amount (if you're paying bills or tuition fees, for instance).
To avoid that, just make sure that the lock icon is activated in the "Recipient gets" field to enable the Amount Lock feature.
With a transfer that has the Amount Lock feature enabled, we will ask for an extra 3% deposit just in case the rate fluctuates. This way, we can make sure that your recipient receives the exact amount they need. We will refund all of the left over from this deposit once your transfer is completed.
If the rate drops slightly between the time when you create a transfer and when we receive and convert your money, we will use some of the extra 3%. If the rate improves during this time, we will refund the 3% (or maybe even more depending on how much the rate changed). We will always return anything that is not needed to convert to the exact, specified amount. You can read more about the amount lock deposit here.