OnStar data is harvested from the monthly diagnostic emails. Average monthly temperature comes from The Capital Weather Gang of the Washington Post which posts the average monthly temperature for National Airport.
I only drive locally, rarely use climate control. So, any differences can be attributed just to temperature changes. I have gotten more efficient with my driving after year one. This can be attributed to three things.
1) raising the PSI eventually to 50 in my tires.
2) extensive use of neutral
3) becoming more obsessed with getting a "good grade" in the monthly emails