Lower octane is just silly, period.. though I am sure you don't want anyone to preach to you. You will get less mileage, and less power with lower than premium. Also, most people say that they don't worry about using less than premium as the computer will "simply" retard timing, etc, effectively relying on the knock sensor to not allow detonation from using an octane that is too low.
One thing about that is: you have to rely on the knock sensor. the sensor doesn't do anything until it has ALREADY detected knock/ping. Not good.
You are likely seeing headwind/tailwind.. elevation climb/elevation fall variances. unless you have performed this trip multiple times, and used premium from A to B once, then from B to A the next time, etc. (this eliminates the elevation change, temperature, humidity, pressure, and wind variances because you are using both grades in both directions...)
Simply put: you either had help (weather, elevation, etc) on one way, or you were driving slightly different in high expectations. Of note: you were likely causing damage the entire time.. joy joy!
Ray
