I tried making similar plots with my Honda Element over a 4 year period when I lived in Florida. I did the same with my Mustang before that. They ran pretty much the same trip every week, work, school, not much else, since we took Sherri's car for most other trips. There were a few outlying bins, The outliers were mostly on the low side and I attributed them to the occasional trips when my 3 mile work commute took 45 minutes, road construction, accidents, and winter tourist traffic.
A friend bought a then new Toyota Camry Hybrid which has much better data gathering on MPG, and gets much better mileage, so small changes are easier to spot. He had a 15 mile commute, and drove his minivan for most of his other driving. He also kept better notes, which pointed out the real reason for the outliers....different brands and grades of gas.
I had always checked the prices of the gas stations on my route and stopped wherever the price was cheaper. He took detailed notes, and figured out his lowest cost per mile (some hybrid owners tend to do that) which turned out to be quite dependent on the brand of fuel. I believe Chevron was his favorite, but every geographic area gets a different blend, which also changes with the season. Even if you go to the same gas station every day, their blend will change with the seasons, particularly in areas with temperature extremes, or areas prone to extreme changes.
In south Florida, bulk fuel comes into Port Everglades, or the Port of Miami. All major delivery tankers start out with the same bulk fuel regardless of which brand of fuel they are delivering. Each brand gets its own unique additive package, which may or may not be fully or properly blended in. This leaves ample opportunity for variation.