I have no certain knowledge, but I’ll offer a hypothesis: I would expect that the plane’s fuel consumption at takeoff and landing (can’t forget that landing also requires fuel) in cold weather – controlling for all other factors, including wind speed and direction, humidity and precipitation, runway surface condition, etc. – would be lower than it is in warmer weather. Cold air is more dense than warm, so it seems to me that the plane’s wing surfaces would be more efficient then, leading to lower consumption.