Welcome to the wonderful world of the electric car!
For most of us, it’s the first time we’ve ever driven a car where the primary fuel tank holds only "a few gallons’ worth of driving distance." This limitation makes us unusually attentive to fuel consumption as we drive (when’s the last time you knew exactly how many miles you had just driven on the most recent quart of gas you had burned?) and estimated remaining range.
Just remember that the fuel tank holds units of fuel, not units of driving distance. It’s up to you to drive efficiently to maximize mileage... and the more consistent your driving habits, the better the range estimate will reflect how you drive.
The numbers on the display can be mesmerizing if you don’t stop to consider the scale of the information. The distance you can drive on one unit of fuel - 1 kWh - can vary a lot depending on the terrain. For example, I live just under 4 miles from a grocery store, and at a slightly higher elevation. On the drive to the store in my 2012 Volt I average 6.3 miles/kWh. On the way home, that drops to 2.2 miles/kWh.
Your vehicle’s computer gathers data about your driving habits as you drive. It then creates an ev mileage estimate, multiplies it by the usable power remaining in the battery, and displays an ev range estimate.
The full charge, start of day ev range estimate is based on the data the computer has gathered about your driving habits, weighted for the most recent driving. Over time, the gathered data will reflect how the vehicle is normally driven (multiple drivers may have differing driving habits, so the more efficient driver will drive further than the less efficient driver on the same amount of available power). Change in weather will affect fuel consumption, so the full charge range estimate tends to cycle up and down throughout the year, just as the temperature does.
And, not to forget, the battery provides fuel for more than just the motor... lots of winter cabin heat or lots of summer air conditioning can reduce the amount of power available for the motor...