I have the feeling that the basic specification for petrol changes from summer to winter to summer and is therefore seasonal as well.
Possibly, but I know that the car coasts further in the summer than in the winter. On my regular trips I do elongated gear changes. By this I mean that as I approach a junction or roundabout, instead of changing down right at the hazard I will select neutral, 50, 60, 100 yards early, and coast up to the point where I select the gear for the hazard. This saves me having to brake. I know exactly when to select neutral to arrive at the hazard at the required speed, but in the winter these points are considerably shorter than in the summer.
I think it is down to the tyres being cold and hard, the transmission cold and stiff, possibly even the road surface (there is a huge difference between wet and dry tarmac as well).
It takes at least 10 - 15 miles of winter driving before the car loosens up and my ScanGauge E begins to show my average journey mpg improving.
Another thing I make great use of is
Deceleration
Fuel
Cut
Off, where the engine uses no fuel at all. The ScanGauge E displays that too. Sometimes, depending on the vehicle speed, 5th gear doesn't give DFCO but a change to 4th does. Every little bit helps in achieving good mpg.