Seasonal Fuel Economy Variance

Here is one year of gas consumption data from my daily commute trip.
Chart shows how winter affect consumption: fuel economy is better in warmer months; more than 1 l/100Km in my test case.
It's a 9+9Km trip to work and back: a longer trip will lead to better and more consistent fuel economy results.
Anyway, this is a real-world scenario: car was between 20.000 and 30.000 Km while these data was collected. I expect even better results when the car will have more kilometers under its wheels.
Time will tell.