what is happening at bitstamp is very fishy.
Didn't you think the KYC was fishy enough? These are the signs of an exchange in trouble. It has happened before, and will likely happen again. You have all been through it before, but can you act on it? Time will tell. My coins have left stamp for now.
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I'll try to answer here: we used to include Bitstamp orderbook into our own orderbook for liquidity purpose. In other words, we were arbitraging between our own platform and Bitstamp. However as we continue to grow we decided to slowly (and I insist on "slowly"
withdraw this arbitraging program to let people do the arbitraging themselves. This is why you started to see a difference between the price on our platform and Bitstamp, as arbitraging for third parties is interesting only after a certain threshold (around 1% like now is not a very good opportunities).

http://www.reddit.com/r/BitcoinMarkets/comments/27vq6b/why_the_large_difference_between_bitstamp_and/ci533wg