Some folks on this forum have talked with Subaru engineers, who note that there is no performance difference between 87 and higher octane gas with the Ascent, because the Ascent hasn't been programmed to take advantage of it. There's also another thread on this forum from the folks at Cobb tuning. They are creating a new tuning profile for the engine, and they noted that the factory default tuning profile will not take advantage of any difference with higher octane fuel.This is funny what you are saying to me))) gas grade does play a role with almost every engine - and this is a fact! put bad gas in and put good gas in and test - then talk here. Theories do not always work in practice! You can Prove if you can by sharing the link for the study or smth. But I was sharing my personal experience, unlike you, basing it on whatever and I saw that premium gas gave me 40+ miles more than regular. Believe it or not, I don't care, but this is a fact for me! FYI my car is brand new 1500 miles on it only and my mileage went from 13.4 to 19mpg as of yesterday. Also, I did the reprogramming thing yesterday also, but it is not for this thread.
If your car has only 1500 miles on it, it may still be breaking in and that can account for a difference. Try switching back to 87 octane and see if you can reproduce the results. Be aware of factors like temperature, weather/wind, driving style, traffic, elevation, etc. could play a part in differences as well.