It isn't winter fuel IMO, because all the stations will be running similar fuel blends.

The single biggest variable in MPG is the driver. 20% variation is easily done by most drivers, alternating between a heavy right foot, and driving easily.

If you are driving consistently, I would ask whether you always fill up at the same station. Different pumps will click off at different points (unless you top off each time yourself).

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue