On a hunch (thanks, Vette), I checked the receipt and even went back to the store to validate the pump number. Turns out I was charged for someone else's lesser amount.
Did a recheck against the actuals and reality is that today's ~100-mile junket saw me running just a tick above 17mpg.
I actually prefer having the real data over some ridiculous, mystery MPG figure.
Now I can sleep.
-V