Futures contract has TIME value, so they are usually higher than the SPOT price, please check this link for detailed explanation .

https://www.investopedia.com/terms/f/futurescontract.asp

Thought it is not exactly same, but similar case is Where you pay Premium with Stock Options to have the right RIGHT to buy the stock one month from now.

Regarding cex & bitstamp they are exchange out side of USA, each of them have different Risk factors associated to each of those exchanges, that is why that price difference .

Coinbase and CBOE are both US based exchanges and are under govt. regulation and are reputable than any other bitcoin exchanges in the world.