Too many variables to consider besides speed.
Bike set up. Side bags/cases, top case, loaded/unloaded, windshield stock/aftermarket, tire type/pressure.
Environment. Temp/humidity, uphill/downhill/grade, road surface/smooth/rough/gravel/dirt/rocky, wind direction/speed.
Fuel. Octane rating/ethanol/non-ethanol/percent of ethanol/water contamination/accuracy of pump.
Were you consistent filling up at a certain mileage/running to reserve/mileage by stock odo or gps? Filling up and checking in short bursts will give a lot of variation.
What is your overall mpg by adding it all up over a longer period?
Gray-haired riders don’t get that way from pure luck.