Winds may have been light, but if you were riding into a(n average) 2 mph headwind one way and with a 2 mph tailwind the other that could easily make more difference than the higher regen.
Speculation, but it's also possible that the first 1% SOC represents more available energy than the last 1% SOC.
In real-world riding there will be some amount of random factors you can't control, like the winds. A bit of a pain, but riding the same route 5 times (for example) would let you smooth some of those random factors out. (though if there's a typical prevailing wind direction, then it would only smooth them out to the average prevailing wind).
Perhaps an easier repeat test would simply be to ride Normal on the outbound leg, and Sport on the return leg. This should balance out prevailing wind, higher SOC to start, etc etc.
With all of that said: I'm curious why Sport appears to be more efficient as well.