I think this is because the difference in buying momentum that occurs on one exchange does not immediately change the price of all exchanges. If large purchases occur in an exchange, the price on the exchange will rise significantly locally and it will take some time for the data to update globally. But this price difference does not last long, but there will still be a slight price difference. This is what is utilized by a certain group of traders called Arbitrage. They use this price difference to make a profit.