avg. mileage screen displays clarification

So I reset my trip odometer to zero after I filled up with premium (I want to see how much it improves gas mileage, compared with 87 octane). I literally drove from the gas station to my house: 11.2 miles. Most of it was highway miles, which explains the Fuel Economy/Average reading/display of 50.2 mpg. However, when I shut off the car, the Drive Info/Avg. reading/display showed 34.9 mpg. I would expect both to be the same numbers for this first trip, where I reset it back to zero. And it was definitely reset, as I witnessed the dash lines that displayed whenever you do that. What am I missing here?? Thanks!