They lose range because the air is denser and requires more fuel to maintain stoicheometry, and because the vehicle spends longer in warm up mode, as well as more basic factors such as thicker lubrication, rolling resistance on tires from either snow or the rubber itself, etc. 

I have a Focus, and as use for a runabout (in Winnipeg) it drops from about 8l/100 in summer to 10-12 in winter, sometimes even worse if there are a lot of unplugged cold starts. When it's -25 out the car loses so much heat through the defrost that it never reaches operating temperature and goes to lean mode, resulting in atrocious mileage.  If you only drive a truck, you may not even realize this is an issue for small cars in the same market this Chevy is in.