The figures are real figures continually submitted by real drivers driving real cars on real roads. If there is a significant difference between summer and winter you will see the average figure dip by the end of the winter. But, having kept proper records of about 25 cars over the years, I haven't seen much of a dip.

It would be greatest for drivers driving short distances from cold because then their engines would not heat up and would be on enriched mixture almost all the time. We saw a similar dip to yours in an Auris HSD (same drivetrain), but that was because the length of journeys the car undertook reduced as the driver got older.