I've noticed this on all vehicles I've driven, which in NE, we get plenty of time to test that theory. We'll need one of our resident mechanics to verify any of this, but what I think it is,
The sensors that manage the amount of fuel that gets injected run off of sensing how many molecules of oxygen are coming into the system as well as exiting. In winter, the molecules are smaller and tighter, therefore, more can fit into the intake tube and into the fuel system at a single time, therefore telling the system to inject more fuel to run the proper A/F mixture. It seems to me that at about 60 dgrees F is the perfect running temp for my truck on 89 Octane.
i think the winter grades of gasoline have something to do with that as well. that and warmin up a vehicle a little in the morns dont help but i hate window scrapin so ill suffer the worse milage... on a side note is that if i was where it was -25f like in that pic stump posted id have to relocate to a better gas milage area. good lord thats cold !!!!