Up until yesterday, the price at Mt Gox was virtually reflected on BlockChain within pennies. Starting yesterday, and still today, there currently is a ~$11 USD difference in price, with the Mt Gox rate being the highest.
I have never seen this before. One or two dollars I may be able to shrug off, but a double digit discrepancy is mindblowing.
I understand weighted averages to this extent: If there were only two exchanges that were being weighed, and one is reflected as being $11 higher, than the other must be $11 lower to see what I'm seeing.
What exchanges are being weighed that's making the Blockchain show such a big difference, given that up to less than two days ago, we were talking pennies, not double digit dollars?
Bitstamp is much cheaper and it gets more business than Gox, so that's probably why. Especially with the inability to withdraw from Gox, Bitstamp is taking the business. Maybe Blockchain just updated their weight with the most recent figures? It was only recently that Bitstamp overtook Mt Gox.