Just like everyone said , the main reason why every other exchange set their own rates is because for the profit. Not only for the profit but it does also depend on the service , let say a high end online exchange will charge a little bit expensive because they set a high fees so that they can send and recieve your coins more faster when compared to regular online exchanges .
of course every exchange has a different calculation, and with different targets. so the set rate will be different. i think we as traders actually have many choices that are in accordance with our desire to use an exchange
but the price difference is not too far between exchanges,
if arbitration is carried out we must think of the amount of withdrawal fees on the Exchange.