That's defiantly inaccurate, guess their feed got a surge in purchases.
EDIT: Looks like "LakeBTC" is our culprit, current price of BTC on their website is $675 which has boosted the Coindesk average to $420 or so.
This is one reason why it is not good to look at these kinds of averages. It would be much more beneficial to look at the trading price of multiple exchanges, along with the volume of each exchange) as it will allow you to identify outliers in price. It would also allow you to identify which exchanges you should avoid (when a certain exchanges price is very different then other exchanges, as this is an indication the exchange is going through some level of problems