Lower tire pressure reduces mpg
Fuel company changes "winter mix" reduce mpg
Cold air reduces mpg
Winter conditions usually mean you end up not driving the same, even if you can, other drivers tend to force you to drive less efficiently
Defrost runs the vent system compressor, same as running AC, this costs you several mpg (running AC does this too, but if you're going from fall, not running AC, to winter, running compressor, this would represent a loss)
That said, others have noticed various actual problems that can crop up in the car, having nothing to do with weather or season, that drop mpg, and you may want to look into them. You might for example have a cracked intake tube.
On the gripping hand (it's an SF joke) miles per tank has it's own problem; pumps can fill up to a arbitrary level (you might fill up to 90% full one day, and to 99% full the next) both of which register full on the gauge, and the gauge itself swings wildly, I've literally seen my gauge go from 1 bar back up to 3 bars. So you can't tell if "Full" is the same as your last "Full" and you can't tell if "Empty" is the same as your last "Empty"