BOTH of those numbers are "estimates" based on many factors but mostly how the vehicle has been driven/operated in the last several cycles. If the car was driven in CS mode in COLD weather it will use that as a baseline to GUESSTIMATE future range.
The major issue with the Volt's ICE fuel mileage is that most of it's use is in COLD temps and often when the car is driven a short distance, parked and driven again. ERDTT will kill fuel mileage.
I suggest filling the tank with fresh premium fuel and taking it on a long drive and watch what happens to those numbers.
There is nothing wrong with your car.
The major issue with the Volt's ICE fuel mileage is that most of it's use is in COLD temps and often when the car is driven a short distance, parked and driven again. ERDTT will kill fuel mileage.
I suggest filling the tank with fresh premium fuel and taking it on a long drive and watch what happens to those numbers.
There is nothing wrong with your car.