refined fuel for many places comes from the same refinery. No difference in base fuel. Some retailers add lubricating or cetane boosters.
Improved mileage is in the density of the fuel, not additives. Summer #2 Diesel is heavier, denser than # 1 winter thinner fuel with its low gel point and easier low temp start .
This is the time of year for northern refiners and sellers to have some winter #1 blend fuel left in their tanks, transitioning to #2 summer diesel with its better power and mileage. Maybe you got some of that.