[RedDevil]

I took my car this sunny morning to my inlaws, a 140 km journey.
Got a trip average of 4.7 l/100 km, trailing semis for about 20% of the journey.

Went back home this evening, same temperature, just with my daughter and some luggage as extra load, and the lights on. Same route the other way. Got 4.1 liter per 100 km.
No trailing this time, and a higher average speed due to keeping up with traffic / not getting in the way. Yet a better economy by more than 10%.
What mystery factor can suddenly improve my economy by that much?

No mystery really, it was the wind. I had a 4 Bft head wind this morning and a tail wind this evening. Just the wind can have this big an impact even while all other factors are adverse.

Even though logic seems to validate your story, it isn't proof unless you can rule out wind and all other circumstantial factors, an impossibility in real world conditions.