Foreign currency values fluctuate not just on a daily basis, but down to the second. When OFX uses the term ‘Market Rate’, we are referring to the Interbank Rate--the rate that banks use when they trade currencies with each other.
The Interbank Rate changes minute by minute. When a bank publishes their buy/sell rate for a currency pair (usually updated daily), they often add a margin of up to 5% on the Interbank Rate to hedge their risk. At OFX, by not publishing a firm daily rate, we can adapt to minute by minute changes in the market, and that allows us to make your transfer at a substantially lower margin.