Also, in winter the drive in the city is MUCH slower, more frequent & longer stops at red lights, plus some idling. This on top of different fuel. So it could add up to 20% or better.

P.S.: plus the car drives longer with the engine at non-operating temps until it fully warms up.

__________________
Stable: e92is, e46 M54B25, e83 N52, e53 N62 - sold, e39 M54B30 R.I.P.