All good comments above. A fully charged Volt shows 10 green bars on the driver’s display. Your 2012 Volt has just a tad over 10 kWh of useable grid power stored in the battery when fully charged, or ~ 1 kWh for each green bar. That power is used not only as propulsion fuel, but also as fuel for the accessories. Cabin heat and air conditioning can gobble up power; less power-hungry items are the radio, the heated seats, the windshield wipers, etc. Power may also be used for battery maintenance when you’re not plugged in. The more of that 10 kWh of power used in your 2012 Volt by the accessories, the less available for propulsion.
It’s normal, then, for your full charge ev estimated range, and your miles/kWh "electric mileage," to cycle up and down throughout the year. In warmer weather, a greater proportion of the power gets used as propulsion fuel by the electric motor. In colder weather, not only does the portion of the full charge used for cabin heat and headlights and windshield wipers usually increase (i.e., weather is colder and rainier/snowier, windows get foggy, nights are longer), but also it takes more electricity to move the car the same distance when the air is cold and dense and the pavement is wet than when the air is warm and the pavement is dry. In short, even if your driving habits remain consistent, the distance you can drive on a full charge cycles throughout the year.