has to do with their signals/functionality, and perhaps a difference in the wiring harness?
I wouldn't use your gas mileage as a benchmark as mine is all over the place too, depending on how many start-ups and shut-downs (short trips) vs. long highway trips. I can usually drive 100 miles before my gauge reflects using the top 1/4 of a tank. However sometimes I'll only drive 70 miles and I'll be close to that same mark.