There are a few other factors to consider.
1) In cold weather the body needs to burn fuel just to keep warm - this impacts performance and also requires more stops.
2) There is an optimum temperature for muscle performance on a bicycle - most of the studies I have seen look at extreme efforts (eg 25 and 50 mile time trials) where the athlete generates more heat - and (from memory) conclude on about 12C - but the temperature would IMHO be higher for endurance cyclists - operating at a much lower intensity. The ideal temperature varies between individuals but my legs work best at about 20C. Nearly all of Steve's riding has been at temperatures much lower than ideal so his legs will not work optimally.
3) Extra clothing, particularly tights v shorts, no matter how stretchy modern materials are, exerts drag on the muscles as they work. So when Steve can ride in shorts he will get a small performance boost.
4) It is more mentally tiring to ride at night, when there is less perception of speed and more concentration required - there may also be brain chemistry factors going on. As the days get longer and Steve spends less time riding in the dark, he will get a speed boost.
I can't speak for Steve, but based on my personal experience, there's between 1 and 1.5mph extra speed in the summer from the factors above. (Stripping out other factors such as a tendency to do more miles and therefore be fitter in the summer).
