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
I think there is no such different values of bitcoin. It is one but yes, the exchanges are always showing it different. Some include their profits and they often manipulate to capture big audience.