Does anyone has a explanation why we did have a 15-25$ spread between the western and the asian exchanges in the last 2-3 weeks?
Haven´t we been mostly equal before?
This is a plot of the ratio (Huobi price)/(Bitstamp price) daily at ~19:30 UTC, when Huobi volume is usually minimum:

The bitcoinwisdom (and other?) charts apparently use the official currency exchange rate 6.21 CNY/USD to compute the equivalent USD price. However, as the plot shows, the actual ratio maintained by arbitrage is around 6.10 CNY at Huobi = 1 USD at Bitstamp. It may have to do with Bitstamp trading or withdrawal fees, or other factors.