My other car is a POS too
Very long term. There's seasonal changes, different winds, temperatures, winter fuel vs summer,You ever hear of something called an "average?" Of course not...
A 90- day average gives you a good handle on what a change did for you. Long term averages smooths out those bumps in the data.
Tires get smaller as they wear, etc.
You're the one who decided to nitpick this anyway. 🤔