I had the same problem the first couple of months when filling up my tank, the expected range would always be 400-435 km more or less.
Then I realized that I was always filling my tank with 1/2 or 1/4 of gas in the tank and never when the range would reach 0 km. Therefore as an experiment I started filling my tank when the range would show 0 and drive on what "supposedly" is an empty tank for about 2-5km until re-fuel.
Now as an approximate, I know that in order to fill my tank as per my last 4 refuels I need 44 Liters of gas (51 Liters is the max, as per user manual, therefore 7 Liters are in reserve not calculated by the range algorithm) so after filling my tank from 0 km my range jumped from 435km to 640-680km!
So in brief, if you want to see your maximum range, wait until your range reaches 0, fill in the tank and see what is the new range number.
Real life example: After re-fueling I would see a range of 640km, do a trip of 150Km, and my range would show 670km afterwards (Driving not over 90km/hr). The algorithm analyzes the speed and distance you are traveling and what has been the consumption. Since every drive is a new drive the algorithm accommodates to that specific driving trip only and sets the range accordingly. From my experience after refueling if you drive properly and do not floor the gas, the range will increase, but if you floor the gas, the range will decrease rapidly!
I would also say as a humble guess, that the range stat is completely reseted when it reaches 0 and not while it still shows some range on the panel , therefore again as a humble guess IMO the range algorithm has already predetermined parameters which calculates the range when refilling completely the tank not taking into consideration your average mileage/ kilometers from previous drives.
To conclude, your range will depend mostly on your driving behavior / habits. The wiser you use the gas pedal, the better your range stats will be.
I hope this helps and that is was clear!
P.S: This is my own experience and this can vary from driver to driver.