Calculating the price of oil

Question

So I've seen a lot of questions about the oil crisis, but what I still didn't figure out is how the broker (providing you the ability to buy commodities) calculates their prices in respect to contracts.

I have an account on XTB. When the May contract was ending the oil took a dip and I bought some for 7 per barrel. This dip can be seen on their graph https://www.xtb.com/en/trading-services/range-of-markets/commodities-trading/oil-wti. But the May contract for oil went negative and June contract was around 10 bucks when I was buying it for 7, so where the price of 7 bucks came from?

I've read that they use something called rollover but if I understood correctly that only means that when the contract expires it automatically obtains you a new one and recalculates your holdings accordingly.

Answer 1

So I've kept digging and I think I've finally found out. Every broker has their own rollover tables (if they provide rolling over for clients) and they also can take extra measures to choose which future contract for commodity (probably they can even provide mixture of contracts in some ratio), they will provide.

In short it seems that every broker can choose and provide any future contract of the commodity or a mixture. In case of XTB, the whole price seems to be bound to July contract meaning that what they are currently offering is Oil WTI July future. When time will get closer to expiration, they will sell/buy those contracts (depends if you are buying or shorting) and exchange them for newer once (next months) for you and recalculate the prices.

But maybe I've misunderstood so take this with a huge reserve! This is just my take on it.