Gemini is the only exchange that has an official fixing rate. Just mere speculation here, but I suspect potentially in their rush to beat CME to market, they went with Gemini as it was the only readily available fixing rate. That and someone coming from traditional markets and accustomed to market efficiency may not be too familiar with just how fragmented and thinly traded crypto exchanges are. We trade Bitcoin in Gemini and have seen how even modest sized trades have to be planned out carefully so as not to move the market too much.

On the other hand, the CME Bitcoin Futures seems to be a bit more thought out. They are using standardized reference rates they created in November 2016, which have independent oversight (CME CF Bitcoin Reference Rate & CME CF Bitcoin Real Time Index). CME’s rates pull from a number of exchange sources such as Bitstamp, GDAX, itBit and Kraken, making it harder to manipulate.