BTW, when the weather changes, gasoline refieries reformulate the fuels for the local weather conditions. (I just rembered that)
They change lots of stuff to control things like Volatility and Reed Vapor Pressure (RVP), things that I used to know all about but have long since forgotten....
That can also slightly affect your fuel mileage. That and some of the things mentioned so far can take big bites out of the numbers.
Altitude and weather play a role as well.