The more liquidity there is on the exchange, such as if a lot more people trade open.btc, it will come to match the Binance price. The difference is due to lack of volume to make it keep liquid and matching outside exchanges. On the other hand, good opportunity for arbitrage can exist by taking cheap UIAs of DEX, withdraw to Binance and trading there. Just make sure you know what you are doing, which way around and in and out of DEX/CEX you go, as that low volume means trades can take much longer..