The range estimate just tells how far you'll be able to go if you keep driving like you've been driving recently.
If you've recently driven down a huge hill, it might say 35 miles, and would probably be right - if you somehow had another huge hill to keep driving down, you could go 35 miles. But you're unlikely to have that much hill, so you probably won't go that far.
If you've just started up on a cold day and the heater is cranking, it might say 10 miles. And if the heater kept cranking full blast, that's all you could go. But, the heater power tapers off as the car warms up, so you'll probably go farther.
Edited by viajero, 02 April 2014 - 05:56 PM.