ICE cars, at least some of them, have range to empty calculations as well, and they are often wrong, too. The computer does NOT know how you will drive in the future or the weather, so it is an educated guess. You'll find that your estimate is better IF you're running the nav system with a destination in it, as then, the car DOES know your intended route, speed limits, and terrain, so its estimate is more educated.
AFter awhile, you'll get an idea of your typical miles/kwHr, and can estimate your range if you can do a bit of math in your head. Given the moving average the i3 uses, that may be more accurate if your driving is going to be different and/or in different environmental conditions.
Everyone's driving technique is somewhat unique. The computer does a decent job, just like the gas gauge on an ICE about how much is left, but how far you'll get, only you can figure that out. In general, the car's estimate is a bit pessimistic, and you can eke out a few more unless your changing your techniques.
2011 535i x-drive GT, 2014 i3 BEV