Sounds good, as I don't dispute your wisdom. Just having done tank after tank of mostly premium through MO & AR (no ethanol) my milage surged by 4 -6 MPG when calculating from milage and receipt gallons. When
premium wasn't available, noticed a lower MPG. Normally I ride in IL/WI (with ethanol), with very consistent MPG. So, what?, elevation, latitude... I'm very "fastidious" about how I fill each time, going for a max tank filling. Thanks to the GPS for finding a gas station with ~5-mi left in tank (had 5-miles to go when BC went "---"). Don't underestimate the distance between gas stations in AR.
Without knowing your circumstances, I can only guess what what may have caused the mileage difference. However, I can say with certainty that it wasn’t the octane of the fuel.
I haven’t yet got to entering my fuel purchases into my spreadsheet, but on my recent trip to Alaska I had MPG tank averages (going by the BC) that ranged from a low of 38 to a high of 50. The biggest factors are wind, speed, elevation change and temperature.
If I cruise at 70 in calm air, 70+ degrees, and level road, I get about 42 MPG. If I cruise at 55 in calm air, 70+ degrees, and level road I get about 50 MPG.
If I cruise at 70 into a 15 MPH headwind and temps in the 50s, I get about 38 MPG. I got 32 MPG a couple years ago cruising on the level across Minnesota at 80 MPH into probably a 30 MPH gusty wind that was varying from 20-40 degrees off the nose.
I bought gas at one “no name” station in Canada and was getting about 4 MPG less than normal. I was thinking I had got some gas that had more than 10% ethanol, then I came to a series of steep descents and realized that I had been gaining altitude very gradually and imperceptibly for probably 30 miles and that almost certainly accounted for most of the MPG difference. After I got to the bottom of the descents, the average was nearly back to normal.
Even in normal conditions, I vary 1-3 MPG tank to tank just due to differences in fill level, temperature, traffic, etc. Unless you ride the same route every day in very similar weather and very similar traffic and park in the same spot at the same pump at the same station, it is nearly impossible to have less than 1 MPG tank to tank variation.
I have every fill-up I have made in my bike since it was new in a spreadsheet, so it is very easy to see the variation and it is surprising.