I'm feeling the concern like rest of you (the famous range anxiety). The keyword here is "estimate". The battery didn't get more capacity over a software upgrade. Tesla improved motors efficiency, thus we are getting more mileage.
For example, your driving habit have been using 250 wh/mi, the 40.8 update might lower it to 245 wh/mi.
72,000 usable battery capacity / 250 = 288 miles
72,000 usable battery capacity / 245 = 293 miles
For my own personal test, I estimated to have ~71kWh usable capacity when I first got it, but I have seen others have 72kWh