Diesel can't be compared with gasoline MPG. One barrel of crude oil, when refined, produces about 19 gallons of finished motor gasoline, but only 10 gallons of diesel.
One barrel of crude oil can be refined into 19 gallons of gasoline and 10 gallons of diesel. In addition to a bunch of other useful petroleum products. Its not like those 19 gallons of gasoline are somehow equivalent to 10 gallons of diesel.