Cold weather and winter driving conditions can reduce your fuel economy significantly. Fuel economy tests show that, in short-trip city driving, a conventional gasoline car's gas mileage is about 12% lower at 20°F than it would be at 77°F. It can drop as much as 22% for very short trips (3 to 4 miles).
Could also be a combination of small things. These can add up. Gasoline differences - Winter Grade RVP vs Summer Grade RVP. Snow vs. Summer Tires, Tire Inflation. Driving Style. Weight (more passengers/cargo) . Aero inefficiencies - open windows, roof rack, cargo box, etc). Road Conditions (snow, rough roads, etc.). Electrical (poor grounds, connections, voltage, etc). Changes in oil brand, type, viscosity.