Neither: Its not called the GOM for a reason (GOM = Guess O Meter).
The mileage shown at full (or any other time for that matter) is based on your driving style and the roads you've taken. If you drive on the highway a lot and then drive "a tank" on surface streets going slow the next "tankful" will show much more miles then drive on the highway again and the next GOM value at full is lower.
You can watch this: Drive on the highway a lot and then find a long stretch of road thats 45 mph or less and drive that with the cruise...you'll watch the GOM value creep up instead of down.
Now other things also affect the GOM's value, like outside temperature, or HVAC setting (turn it on/and off with the heat set to high and watch the GOM value jump up or down at least 10 or 20 miles just by turning the heat on or off).
In addition: When its colder out the car is going to turn on a heater to heat the battery this reduces the driveable range simply because its using some electricity to heat the battery.
Your battery's real reading is the % value..not the range to empty as that is..... just a guess.
2018 Cajun Red Chevy Bolt
2016 Magnetic C-Max Energi (lease returned)
2014 Sunset Escape
2014 Thor Axis (V-10)
2013 Focus Electric (lease returned Jan 2016 for the C-Max above)