There have been articles saying that the Chevy Volt is not a hybrid because the IC engine is never connected to the wheels.
Wrong- it is a series hybrid. There are two varieties of hybrid, the parallel and the series hybrid. The parallel hybrid usually has the IC driving the wheels, electric motor boosts when needed.
The series hybrid has electric drive only, IC only charges battery. The series hybrid maximizes the IC engine efficiency, but the overall system efficiency may be lower, as it requires a larger electric motor and battery, which increases the weight compared to parallel hybrid.
So the final milages are close, and depends on the details and skill of designers.