The answer is arbitrage.
Tell me how you'd do it, please.
Would this work:
* Buy BTC with USD outside China
* Sell the BTC in China for RMB
* Use the RMB to buy goods or stock in Chinese market
* Sell goods/stock outside China for USD
or the other way around. Not for amateurs, of course.
This would equalize prices but according to some "exchange rate" that is not the official one, due to extra costs.
Which is what we see, right? They track each other but not at the official exchange rate.