OKcoin futures index calculation problem

I have been confused by OKCoin's futures index price for a while. I hope OKcoin representatives can step up and answer it below.

As of now, and I quote from their calculation method as follows: "Currency Conversion: The chosen unit of account is USD. All non-USD prices will be included in the index after being converted to USD. The currency rate shall be converted using the official exchange rate released by the Chinese Central Bank every morning at 9:30 am CST (UTC+8)." Source for calculation

The exchange rate published today from Bank of China: (http://www.boc.cn/sourcedb/whpj/enindex.html)

Today's rate, although not at 9:30AM CST yet is 6.2585 RMB to USD, (buyrate+sellrate)/2

Yesterday's rate, is 6.25RMB to USD

Currently index price: 241.33

  • Huobi: 1494.92

  • OKcoin: 1491.56

  • Btcchina: 1491.99

  • OkcoinUSD: 239.22

  • Bitfinex: 240.43

  • Bitstamp: 237.7

With these numbers and the published bank rates: The index calculation is: 238.79. The discrepancy between the index and what i've calculated is off by more than 1%. If I were to trade the 20x okcoin futures product, I would be pretty pissed if it settled 1% above what I've expect.