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Getting back on topic, this has nothing to do with measurement systems. As Dave pointed out, it is all about the quality if the crude...or in this case a synthetic crude derived from the mining process. They are not equal in quality and therefore not equal in price.
A barrel of oil is also priced according to location. Obviously a tank load or rail car full of oil at a refinery is worth more than one at the wellhead as the well head price needs to account for the future costs of transportation.
Regarding quality though, certain crudes can only be processed through certain refineries customized to meet the specifications of the crude. Heavy sour crudes cannot be processed in refineries designed for light sweet (low sulfur) crudes. The sulfur content is important but also other impurities such as nickel and vanadium.
Finally there is the market factor. Crude sold on the spot market (e.g. WTI at Cushing, Oklahoma) differs in price from that sold in the long term futures market. The market prices will be influenced by demand as well as weather. A hurricane in the Gulf of Mexico can change spot prices in other areas as production offshore is curtailed.