•   Valuation Risk — In certain circumstances, portfolio holdings may be valued using techniques other than market quotations. The value established for a portfolio holding may be different from what would be produced through the use of another methodology or if it had been priced using market quotations. Portfolio holdings that are valued using techniques other than market quotations, including “fair valued” securities, may be subject to greater fluctuation in their value from one day to the next than would be the case if market quotations were used. In addition, there is no assurance that the Fund could sell a portfolio holding for the value established for it at any time, and it is possible that the Fund would incur a loss because a portfolio holding is sold at a discount to its established value.
Source: MV Index Solutions GmbH (MVIS®). MVIS is a wholly owned subsidiary of Van Eck Associates Corporation. Data as of December 8, 2017 (synthesized data from BitMEX, OKCoin, CryptoFacilities, and BTCC which represents non-U.S. listed bitcoin futures trading on these exchanges). Not intended to be a forecast of future events, a guarantee of future results or investment advice. Current market conditions may not continue.  
The validity of each cryptocurrency's coins is provided by a blockchain. A blockchain is a continuously growing list of records, called blocks, which are linked and secured using cryptography.[28][31] Each block typically contains a hash pointer as a link to a previous block,[31] a timestamp and transaction data.[32] By design, blockchains are inherently resistant to modification of the data. It is "an open, distributed ledger that can record transactions between two parties efficiently and in a verifiable and permanent way".[33] For use as a distributed ledger, a blockchain is typically managed by a peer-to-peer network collectively adhering to a protocol for validating new blocks. Once recorded, the data in any given block cannot be altered retroactively without the alteration of all subsequent blocks, which requires collusion of the network majority.
There's a lot of ins and outs which can get confusing in arbitrage trading. We won't go into the technicals of why futures contracts trade at a premium to spot price. You can read a full explanation here. If it's not of interest to you, all you need to know is that there's a tendency, the further out in time the futures contract expires, for the premium to spot to be higher and higher in nominal percentage terms.