Fuelly computing the wrong MPG... and it's intentional...?

[spiceygas]

I normally fuel up with 91 octane fuel. The other day I bought a tank of 93 octane fuel for the first time (because the station didn't have 91).

When I looked at my app, I was surprised to see that it had already computed my fuel efficiency for 93 octane despite the fact that I had not yet driven a tank of 93 octane through the car.

i.e. The app is wrong. The mathematically correct behaviour would be to associate the MPG computed during the fill-up with the octane of the previous tank.

I put in an issue ticket, and support told me that there "are advantages and disadvantages to both methods of which tank to assign the fuel efficiency to. Years ago, Fuelly decided to assign it to the current tank."

The disadvantage is obvious: the graph is unreliable. (attached) Anyone who wants to know the correlation of MPG with octane is being misled.

I am struggling to see the advantage of their choice. Can someone help me understand?