If I can buy privately (or from Coinbase, etc.), transfer my coin to Gox, and then sell those on Gox for a higher price, then I make money. But because of the difficulty in getting fiat out of Gox, that trade is moot for most of us. Hence the built-in "fear." That is, I'm willing to take a risk by depositing my at Gox only if there's a chance I can withdraw a $200 premium (at today's rate) per coin.
Note: For the BTCChina comparison, I subtract the closing price on each exchange in Yuan, and then I divide the result by that day's USDCNH exchange rate. Thus, the USD equivalent of the CNY gap.