Every trading site has its own buying and selling order book. This decides the price on each seperate exchange. They are in fact not connected to eachother.
I think that is a problem. I had one website with ETH at $315 and another at $324.
It seems that some exchanges are voluntary showing a price lower so that they can pocket the difference!
It's not a problem. Arbitrage traders generally correct these imbalances pretty quickly. They run bots on several exchanges, reconciling accounts on each one to take advantage of the price gaps between exchanges, rather than the volatility itself. Exchanges could do arbitrage, too, and I don't see any major problem with it.
I definitely think some exchanges push their prices higher or lower to run people's stops or margin call their traders, though. That's a problem, but it'll continue to happen on unregulated exchanges. I'm not sure I even trust the regulated ones.