The range is worked out on your driving style/expected distance based on current fuel usage.
If you have got in the car and booted it along with 5 gallons of fuel in it, instant consumption reading in the computer on average 15mpg, you would expect the range to come up as about 50-60 miles remaining depending how conservative it is.
87 miles/3 = 29mpg (your current "average"), lets say due to "harsh" acceleration your average becomes around 15mpg, that gives your 43mile range, continue to drive in that style and then 10 miles later you drop down to 33 mile range (rough maths) so lets say it actually drops below 30 miles range meaning no more range indicated as stated above.
Range is its estimate, fuel gauge is a very good indicator of how empty the tank really is.