My question is why is the rate higher on one exchange than on another?
The price is made by supply and demand. If demand on an exchange is greater than supply in the short term, prices can rise more sharply there than on other exchanges and vice versa. However, this is usually only the case in the short term because there are many so-called arbitrage traders who use price differences between exchanges to buy cheaper on one and sell more expensively on the other. As a result, the price usually adjusts quickly across exchanges.
He should understand that exchanges operate independently and the supply and demand in each ones determines their prices of crypto coins, and the law of basic economics applies in them, so if demand is much and supply is less in an exchange price will slightly increase and if supply is greater than demand in the same exchange price will slightly reduce. Although among exchanges price differences are not much, as you nareted, traders can buy cheaper in one exchange and sale higher in another exchange, thereby creating a near price balance.