Cliff notated interpretation: They put lighter molecular weight carbon compounds into the fuel during the winter. These lighter weight compounds evaporate better to aid the engine in starting and smooth running. The downside is, there are less Carbon-Carbon and Carbon-Hydrogen bonds. The energy in the fuel is locked up in those bonds. When you brake them to form CO2 and H2O they release energy in the form of light, heat, and volume expansion. Thus Winter fuel mixtures need to run fatter in order to get the proper ratio of air to fuel. Add into that the more dense air meaning even MORE fuel is needed when the engine is working hard and you are going to consume more fuel.
Done like it? Switch to E85 and run that year round.