in different EXCHANGES are different traders who set the price
Indeed.
And to say, I find BTC-E overal a better exchange as in price indication.
It's most of time time around $10 lower than Bitstamp in price which is used as source by many sites and people.
That's the beauty of Bitcoin, no one knows the exact price, it's what people want to pay for it.
I remember MtGox that had $100 higher price than all other exchanges

That was just insane!