in general to understand how this works.....
As i wrote, just pure demand and supply.
no buyer is willing to buy at or above spot price. This is frequent in bearish periods and a Seller is willing to sell at a discount speculating further price decline.
General inference is usually a slight premium that is considered cost-of-carry although this was practical in the commodity exch but bcos mkts are mostly in bull periods, more buying demand keeps the premium up and vice-versa when in bear mkts.