I've noticed it too though I'm only on my second tank of gas.
Tire pressure and driving style are the mostly likely factors. Temperature not so much unless you think about it terms of AC vs no AC.
Yes at colder temperatures the air is more dense and the computer will inject more fuel per air volume, but it will be producing more power and thus for the same driving style, you should use less throttle. Net effect should be zero. Now on cold days you may spend more time letting the car warm up which will affect your mpg.

Yes at colder temperatures the air is more dense and the computer will inject more fuel per air volume, but it will be producing more power and thus for the same driving style, you should use less throttle. Net effect should be zero. Now on cold days you may spend more time letting the car warm up which will affect your mpg.