I think you may find the float may be variable in these machines. I too get about a 100 miles from full to empty, 4 gallon with 2 reserve. I just know I have another 25 miles before I am close to being empty. I prefer to never run it with less then a gallon in the tank as reserve. I have found with fuel pumps they will fail more quickly the more they can possibly be run low and starved for fuel do to movement in the tank.
If you want a bit more correct reading you may have to adjust the float level.