It's exponential because you can only ever (at best) double your MPG on the return leg of a trip. If you use 0 gallons on the return leg, you're doubling your distance, but the total gallons consumed stays the same, and so the MPG doubles.

100 MPG on first leg and infinite MPG on return leg != (infinity+100)/2 total MPG.
It equals 200 total MPG.

Same goes for average MPH problems.