The important thing to remember is that there is only one real exchange rate. Sometimes this is called the mid-market rate, interbank rate and spot rate amongst other things.
The real exchange rate is the mid-point between the BUY and SELL rates on the global currency markets and it constantly fluctuates. It’s the rate you find on Reuters, Google, finance.yahoo.com, etc. and the rate that TransferWise uses.
As we use a peer-to-peer system for many of our transfers, we simply take in rate data from these sites and apply it when converting your money. With some currencies we offer a guaranteed rate, but for those that don't have this feature you can set a rate limit.
You can see more information on our dedicated mid-market rate page.