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