It's not just the gas formulation that can cause mileage differences. In the winter season cars take longer to get up to temperature and start operating more efficiently. Also some people idle their cars more either warming them up first thing in the morning, or while waiting for loved ones to come out of the store other establishment while they might stay in the car.
Think about your winter driving habits vs summer ones and see if there are also other things that you might be doing to use more fuel vs summer. Yes, I know that running the AC in the warmer months uses more gas, but the engines get up to temps quicker and become more themally efficient. One last thing as what has already been discussed here on BITOG.......if your car has a direct injection fuel system, fuel dilution will most likely happen more in winter vs summer......so make sure you change oil accordingly.