BMW's battery management essentially only lets you use the middle of the battery's capacity...it doesn't fully charge all of the time, monitors and manages temperature in the process along with cell balancing, then, won't let you actually discharge it fully. What it displays as the SOC is the usable range, not the actual range. WHen it says zero (and will stop), there's really a moderate amount of battery capacity left.
If you haven't downloaded it, there's an e-book on the i3, now in Rev 3, that's free. It's easier on iOS, but I think they've got a version for Android now, too. One interesting item on that is that 10 charges from 90-100% puts the same wear on the battery as one from 0-100%, so keeping it charged isn't a longevity factor. Mine is now four years old, and this morning it told me 87-miles to empty, about the same as when new. Admittedly, I don't have a huge amount of miles on it, but basically plug it in when I get home all of the time and leave it there. It's nice to always have it at full when I decide to use it again.