Location must count for gas quality

I live in south central Oklahoma. I always fill up at Sam's and it is 100 % gas. When I go to Dallas or Tulsa, I buy 10% E and always get great gas milage compared to Sam's in my town. Last weekend I went to Oklahoma City and filled up with Sam's gas with 10% E and the ride home doing 75 MPH I got 51 MPG compared to 43. Drove 65 MPH to a location 35 miles away and was getting 56 MPG on same tank from OKC Sam's. So if your prius is not getting as good as you think it should, try another station. I just hate putting ethonal in my car.