It seems to me that the number displayed on the screen is based on the MPG you got before fill up. For example, if I cross the Cascade Range, on the freeway with cars doing 75MPH+, I get only around 18 MPG in my Limited Hybrid. The little "miles to fill" screen will show fewer miles than if I'm cruising around on rural roads, seldomly going more that 50MPH. I get 22-24MPG and a when I do fill up, the screen gives me a higher "miles to fill up" based on the recent MPG.
I am not sure its very good at math period. Last couple fill ups it said I had around 20-25 miles to empty but only put in 15-16 gallons each time. I said it was average of 19.4 mpg but after filling up still said distance to empty was like 305 miles.
The math for MPG for those two fill ups were fairly close less than 1mpg off so at least that math is getting better instead of the 3mpg if was off before.