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I attended a mini-class on racefuels a few years ago, put on by Union 76.
I'm not drawing any conclusions, just adding some pieces to the puzzle...
Evaporation is a concern on two fronts, but reduced volume from evaporation aside for a moment: The engineer teaching the class explained the lighter chemicals withing the gasoline mixture have the lower vapor pressure, separate out and evaporate first. Evaporation as a result of heat, vibration, or both. Those lighter chemicals, it was explained, are the components responsible for slightly reducing burn rate (increase octane numbers) and keep detonation from occuring. He also explained that underground fuel storage tanks don't vary much in inside tempature, much like a wine cave the earth helps average out the tempurture.
Since those lighter chemicals evaporate first, a race engine can start to detonate and loose power as the result of running on old fuel that has gotten warm and vibrated within the fuel tank, or fuel that has been left in an unsealed can or jug inside a warm race trailer for 3 weeks. He advised for best performance, only race with fuel from sealed cans or fuel that has been stored in the smallest container possible, sealed, in a cool dry place.
In a passenger car, the smarter ECU's sense detonation and increased coolant temp, then retard ignition timing and/or choose a slightly richer fuel map to keep the engine from hurting itself. With reduced power from less timing lead, the driver must give it 'more accelerator pedal' to maintain a given speed. Combined with the richer fuel map, FE goes down as the lighter chemicals evaporate. With a 1/4 tank of fuel, you are running your off of 'the leftovers' in your tank. That's facts mixed with some theory.
Here's what still doesn't make sense to me... on a passenger car, why is evaporation even a consideration? <thinking out loud here> We all have these evaporative closed loop systems... when I stop to fill up and remove the cas cap, the huge rush of air escapes. Unless you open the gascap, those vapors should just recondense and drop back into the fuel and remix, shouldn't they? <thinking more> Maybe the huge rush of escaping air is the lighter chemicals suspended above the liquid fuel, out of liquid suspension, where it does my engine no good, resulting in lower FE?
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