I am trying to better understand the exchange rates used for Bitcoin and Ethereum. I currently use Blockchain for Bitcoin and Kraken for Ethereum. I notice that the exchange rate for Bitcoin is different between Blockchain and Kraken, often by quite a large amount. I have also looked at Coindesk and that too is very different. Is the rate quote just the latest traded on their own exchange? Is the one common central point to get the best and most accurate rate?