The range estimates are to the indicated zero point. And, the range to 'empty' is constantly being adjusted on a rolling average of the last 18-miles of use. So, unless you' are always driving the same route under the same conditions, it will often change. Say your trip starts out on country roads at lower speeds, but few stops then you get on the highway to your destination. Going one way, the country roads at lower speeds would seem to cause the range to creep up for a bit. But, after you get onto the highway and haul along with traffic (speed is the bigger killer of range), it would drop much more rapidly. I've had the range increase or remain the same after driving as much as 13-miles because the power use pattern was different than the previous use.
FWIW, this is no different than the range to empty you get on an ICE. I might see 300-miles to empty on my previous ICE, then part way into that use, get on the interstate, sit at a constant speed for hours, and it might rise up to 500-miles to empty. That's a huge difference, as it can be with an EV.
People get obsessed with the range on an EV since it takes much longer to fill it back up again, and there aren't as many places to do it as with an ICE. It is important, but you need to learn your vehicle. Bigger batteries in general either take up more space, or in the case of the i3 so far, become heavier...weight means range loss, so there's a magic point where bigger batteries can become a detriment as more weight means more tire wear, brake wear, heavier duty bearings, and suspension components, and that extra weight also affects range. Increased energy density is the big thing. A couple of kilos of hydrogen can drive a fuel cell vehicle maybe 300-miles, but where do you refill it from? Eventually, that may become the way we go with EVs, but it takes lots of energy to split water and compress the hydrogen, and until excess green energy is available, not likely to become widespread.
Throw in a large heating need, especially on the REx without the heat pump in the BEV, and then see what your cold weather range becomes!
2011 535i x-drive GT, 2014 i3 BEV
Soon (hopefully!) A 2021 X5 45e will replace the above